'RELENG_7_0_0_RELEASE'.

This commit was manufactured to restore the state of the 7.0-RELEASE image.

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

2. Add command 'thread signal' to print out current thread's signal mask
   and pending signals.

list of frame sniffers so that trapframes can be detected. The kluge
is needed because this version of gdb only supports appending a
sniffer to the list of sniffers and the moment kgdb gets a chance to
add its own frame sniffer, the target's default frame sniffer is
already in the list. Since the default frame sniffer claims any
frame thrown at it, kgdb's frame sniffer never gets to smell (a
process much akin to tasting, but with lesser chance of hurling :-)

This commit adds dummy frame sniffers that never claim a frame and
as such don't fix anything yet. However, we now have frame sniffers
and they are being called, so it's just a matter of adding meat to
the bones and we'll be able to properly unwind across trapframes.

MFC after: 1 week

SSE (or its successors).

Reviewed by: marcel, davidxu
MFC After: 2 weeks

PR: threads/76821