riscv/cmpxchg: Deduplicate xchg() asm functions

riscv/cmpxchg: Deduplicate xchg() asm functions

In this header every xchg define (_relaxed, _acquire, _release, vanilla)
contain it's own asm file, both for 4-byte variables an 8-byte variables,
on a total of 8 versions of mostly the same asm.

This is usually bad, as it means any change may be done in up to 8
different places.

Unify those versions by creating a new define with enough parameters to
generate any version of the previous 8.

Then unify the result under a more general define, and simplify
arch_xchg* generation.

(This did not cause any change in generated asm)

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Tested-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20240103163203.72768-3-leobras@redhat.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>