perf: Move arch specific code into separate arch directory

The perf userspace tool included some architecture specific code to map
registers from the DWARF register number into the names used by the regs
and stack access API.

This moves the architecture specific code out into a separate
arch/x86 directory along with the infrastructure required to use it.

Signed-off-by: Ian Munsie <imunsie@au.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>