first getting the current state with td_thr_getxmmregs_p. Without this,
debugging a threaded app that uses libthr resulted in kernel panics or
spurious SIGFPEs for me.
(As of revision 1.6, sys/i386/i386/ptrace_machdep.c masks off the
reserved bits in the mxcsr register, which prevents the kernel panics.)
Architectures without PT_GETXMMREGS are not affected.
MFC after: 1 week