Stabilize s390x inline assembly

[taiki-e]

This stabilizes inline assembly for s390x (SystemZ).

Corresponding reference PR: rust-lang/reference#1643

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From the requirements of stabilization mentioned in #93335

Each architecture needs to be reviewed before stabilization:

• It must have clobber_abi.

Done in #130630.

• It must be possible to clobber every register that is normally clobbered by a function call.

Done in the PR that added support for clobber_abi.

• Generally review that the exposed register classes make sense.

The followings can be used as input/output:

• reg (r[0-10], r[12-14]): General-purpose register

• reg_addr (r[1-10], r[12-14]): General-purpose register except r0 which is evaluated as zero in an address context

  This class is needed because r0, which may be allocated when using the reg class, cannot be used as a register in certain contexts. This is identical to the a constraint in LLVM and GCC. See Support reg_addr register class in s390x inline assembly #119431 for details.

• freg (f[0-15]): Floating-point register

The followings are clobber-only:

• vreg (v[0-31]): Vector register

  Technically vreg should be able to accept #[repr(simd)] types as input/output if the unstable vector target feature added is enabled, but core::arch has no s390x vector type and both #[repr(simd)] and core::simd are unstable. Everything related is unstable, so the fact that this is currently a clobber-only should not be considered a stabilization blocker. (s390x vector facilities support #130869 tracks unstable stuff here)

• areg (a[2-15]): Access register

All of the above register classes except reg_addr are needed for clobber_abi.

The followings cannot be used as operands for inline asm (see also getReservedRegs and SystemZELFRegisters in LLVM):

• r11: frame pointer
• r15: stack pointer
• a0, a1: Reserved for system use
• c[0-15] (control register) Reserved by the kernel

Although not listed in the above requirements, preserves_flags is implemented in #111331.

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cc @uweigand

r? @Amanieu

@rustbot label +O-SystemZ

[Amanieu]

LGTM, but stabilization needs a T-lang FCP.

[taiki-e]

@rustbot label +S-waiting-on-team +needs-fcp