Rollup merge of #131258 - taiki-e:s390x-stabilize-asm, r=Amanieu

Stabilize s390x inline assembly

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 rust-lang/rust#93335

> Each architecture needs to be reviewed before stabilization:

> - It must have clobber_abi.

Done in rust-lang/rust#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 rust-lang/rust#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. (rust-lang/rust#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](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/SystemZ/SystemZRegisterInfo.cpp#L258-L282) and [SystemZELFRegisters](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/SystemZ/SystemZRegisterInfo.h#L107-L128) 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 rust-lang/rust#111331.

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cc ``@uweigand``

r? ``@Amanieu``

``@rustbot`` label +O-SystemZ +A-inline-assembly