"sandwich" rlibs between native deps in linker order

[krasimirgg]

This is part of the work on #1268.

Consider a case where we have a rust binary B that depends on a rust library R that depends on a native library N. With a recent nightly rustc that passes the native library via -l to the linker (not using --whole-archive as it used to) and a linker that checks for backward references, we can run into backward reference errors when N appears before R on the linker invocation. Example at rust-lang/rust#81490 (comment).

We came up with two approaches to deal with such cases: a "sandwich" one and one where we'd pass appropriate -l flags to rustc when building rust_libraries. This PR implements the "sandwich" one.

Under the "sandwich" approach, we modify the rustc command link for building B in a way that the native libraries section on the linker invocation is repeated once more after the rlib section:

ld ... n1 n2 ... r1 r2 ... n1 n2 ...
        X          Y         Z

By doing this, we allow the linker to resolve a dependency from an rlib to a native lib from Y to Z.

This should allow us to remove the experimental_use_whole_archive_for_native_deps stopgap feature, while additionally allowing us to take advantage of the new rustc no-whole-archive behaviour.

The approach to pass appropriate -l when building rust_libraries also seems plausible, but we've got some concerns about scaling in some cases, e.g., where we have deeply nested "ladder-like" dependency graphs composed of mixed rust and native libraries. Embedding the transitive set of native library names in the rlib also means we may need to build multiple variants of rlibs to work, e.g., in pic/no-pic mode, whereas now a single rlib is sufficient for both modes. We'll continue to investigate.

[scentini]

LG, thanks!

[keith]

This probably fixes #1276 too