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 <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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