sim,arch: Collapse the ISA specific versions of m5Syscall.

sim,arch: Collapse the ISA specific versions of m5Syscall.

The x86 version doesn't do anything x86 specific, and so can be used
generically in sim/pseudo_inst.(hh|cc)

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I46c2a7d326bd7a95daa8611888051c180e92e446
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23177
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>