This commit brings in a number of minor updates for rustc's support for
the wasm target which has changed in the LLVM 9 update. Notable updates
include:

* The compiler now no longer manually inserts the `producers` section,
  instead relying on LLVM to do so. LLVM uses the `llvm.ident` metadata
  for the `processed-by` directive (which is now emitted on the wasm
  target in this PR) and it uses debuginfo to figure out what `language`
  to put in the `producers` section.

* Threaded WebAssembly code now requires different flags to be passed
  with LLD. In LLD we now pass:

  * `--shared-memory` - required since objects are compiled with
    atomics. This also means that the generated memory will be marked as
    `shared`.
  * `--max-memory=1GB` - required with the `--shared-memory` argument
    since shared memories in WebAssembly must have a maximum size. The
    1GB number is intended to be a conservative estimate for rustc, but
    it should be overridable with `-C link-arg` if necessary.
  * `--passive-segments` - this has become the default for multithreaded
    memory, but when compiling a threaded module all data segments need
    to be marked as passive to ensure they don't re-initialize memory
    for each thread. This will also cause LLD to emit a synthetic
    function to initialize memory which users will have to arrange to
    call.
  * The `__heap_base` and `__data_end` globals are explicitly exported
    since they're now hidden by default due to the `--export` flags we
    pass to LLD.

This commit moves `thread_local!` on WebAssembly targets to using the
`#[thread_local]` attribute in LLVM. This was recently implemented
upstream and is [in the process of being documented][dox]. This change
only takes affect if modules are compiled with `+atomics` which is
currently unstable and a pretty esoteric method of compiling wasm
artifacts.

This "new power" of the wasm toolchain means that the old
`wasm-bindgen-threads` feature of the standard library can be removed
since it should now be possible to create a fully functioning threaded
wasm module without intrusively dealing with libstd symbols or
intrinsics. Yay!

[dox]: WebAssembly/tool-conventions#116

This extracts the core visiting logic

… and avoid rightwards drift

They are now library features.

ignore wait-forked-but-failed-child.rs as there is no command 'ps' on vxWorks

ignore process-sigpipe.rs as there is no 'sh' on vxWorks

ignore core-run-destroy.rs as there is no 'cat' and 'sleep' on vxWorks

rustc: Update wasm32 support for LLVM 9

This commit brings in a number of minor updates for rustc's support for
the wasm target which has changed in the LLVM 9 update. Notable updates
include:

* The compiler now no longer manually inserts the `producers` section,
  instead relying on LLVM to do so. LLVM uses the `llvm.ident` metadata
  for the `processed-by` directive (which is now emitted on the wasm
  target in this PR) and it uses debuginfo to figure out what `language`
  to put in the `producers` section.

* Threaded WebAssembly code now requires different flags to be passed
  with LLD. In LLD we now pass:

  * `--shared-memory` - required since objects are compiled with
    atomics. This also means that the generated memory will be marked as
    `shared`.
  * `--max-memory=1GB` - required with the `--shared-memory` argument
    since shared memories in WebAssembly must have a maximum size. The
    1GB number is intended to be a conservative estimate for rustc, but
    it should be overridable with `-C link-arg` if necessary.
  * `--passive-segments` - this has become the default for multithreaded
    memory, but when compiling a threaded module all data segments need
    to be marked as passive to ensure they don't re-initialize memory
    for each thread. This will also cause LLD to emit a synthetic
    function to initialize memory which users will have to arrange to
    call.
  * The `__heap_base` and `__data_end` globals are explicitly exported
    since they're now hidden by default due to the `--export` flags we
    pass to LLD.

…-2, r=Xanewok

Various cleanups to save analysis

…oli-obk

Miri: fix determining size of an "extra function" allocation

Fixes [a bug](rust-lang/miri#862) introduced by rust-lang#62982. Best reviewed commit-by-commit.

r? @oli-obk

…nkov

cleanup: Remove some language features related to built-in macros

They are now library features.
Cleanup after rust-lang#62086.
The unstable book files are moved because tidy complained.

Ignore test cases that are not supported by vxWorks

bypass x86stdcall.rs for vxworks

ignore wait-forked-but-failed-child.rs as there is no command 'ps' on vxWorks

ignore process-sigpipe.rs as there is no 'sh' on vxWorks

ignore core-run-destroy.rs as there is no 'cat' and 'sleep' on vxWorks

…varkor

Update `impl Trait` gate issues

cc rust-lang#63065
cc rust-lang#63063

r? @varkor cc @alexreg