(__HAVE_PTRACE_MACHDEP) and procfs (__HAVE_PROCFS_MACHDEP).
These changes will allow platforms like x86 (XMM) and PowerPC
(AltiVec) to export extended register sets in a sane manner.
* Use __HAVE_PTRACE_MACHDEP to export x86 XMM registers (standard
FP + SSE/SSE2) using PT_{GET,SET}XMMREGS (in the machdep
ptrace request space).
* Use __HAVE_PROCFS_MACHDEP to export x86 XMM registers via
/proc/N/xmmregs in procfs.