bits/signal.h: add register names for x86(_64)
[musl] / arch /
drwxr-xr-x   ..
drwxr-xr-x - arm
drwxr-xr-x - i386
drwxr-xr-x - microblaze
drwxr-xr-x - mips
drwxr-xr-x - powerpc
drwxr-xr-x - x86_64