x86inc: Avoid creating unnecessary local labels

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.

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.

Signed-off-by: Anton Khirnov <anton@khirnov.net>