arch-x86: Change guest ABI for x86 pseudo insts

arch-x86: Change guest ABI for x86 pseudo insts

Change the guest ABI for x86 pseudo instructions to explictly write rax.
This is required because for some reason, the KVM CPU overwrites rax
after the KVM MMIO sets the value.

Note: This is hacky. It will only work for the current implementations
of x86 m5 ops which have their return value in RAX. A comment is added
to the m5ops file to make this clear.

Change-Id: I9466bf050b26db3650cfe3d23008e0f77fda8bc0
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25664
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>