x86inc: Avoid creating unnecessary local labels

The REP_RET workaround is only needed on old AMD cpus, and the labels clutter
up the symbol table and confuse debugging/profiling tools, so use EQU to
create SHN_ABS symbols instead of creating local labels. Furthermore, skip
the workaround completely in functions that definitely won't run on such cpus.

This patch doesn't modify any emitted instructions, and doesn't actually affect
x264 at all. It's only for other projects that use x86inc.asm without an
appropriate `strip` command in their buildsystem.

Note that EQU is just creating a local label when using nasm instead of yasm.
This is probably a bug, but at least it doesn't break anything.