x86: In X86 linux, system calls simply return their result in a register.

x86: In X86 linux, system calls simply return their result in a register.

We can partially define an x86 linux system call ABI which handles the
return value. Argument gathering would be handled elsewhere.

Change-Id: Id2841dea39aa2ea3fc42e1a0fb70ff4e477b3671
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23442
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>