Rollup merge of #119431 - taiki-e:asm-s390x-reg-addr, r=Amanieu

Support reg_addr register class in s390x inline assembly

In s390x, `r0` cannot be used as an address register (it is evaluated as zero in an address context).

Therefore, currently, in assemblies involving memory accesses, `r0` must be [marked as clobbered](https://github.com/taiki-e/atomic-maybe-uninit/blob/1a1155653a26667396c805954ab61c8cbb14de8c/src/arch/s390x.rs#L58) or [explicitly used to a non-address](https://github.com/taiki-e/atomic-maybe-uninit/blob/1a1155653a26667396c805954ab61c8cbb14de8c/src/arch/s390x.rs#L135) or explicitly use an address register to prevent `r0` from being allocated to a register for the address.

This patch adds a register class for allocating general-purpose registers, except `r0`, to make it easier to use address registers. (powerpc already has a register class (reg_nonzero) for a similar purpose.)

This is identical to the `a` constraint in LLVM and GCC:

https://llvm.org/docs/LangRef.html#supported-constraint-code-list
> a: A 32, 64, or 128-bit integer address register (excludes R0, which in an address context evaluates as zero).

https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html
> a
> Address register (general purpose register except r0)

cc ``@uweigand``

r? ``@Amanieu``

Author: matthiaskrgr