x86: Separate system call tables into their own files.

x86: Separate system call tables into their own files.

These tables take up a lot of space and obscure what's going on in the
file around them. This change moves them into their own files (one for
32 bit and one for 64 bit). It also moves the x86 local definitions of
some system calls into their own file, and creates a SConscript file for
the linux subdirectory.

Change-Id: Ib0978005783b41789ea59695ad95b0336f6353eb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34160
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>