debugserver should advance pc past builtin_debugtrap insn

On x86_64, when you hit a __builtin_debugtrap instruction, you
can continue past this in the debugger.  This patch has debugserver
recognize the specific instruction used for __builtin_debugtrap
and advance the pc past it, so that the user can continue execution
once they've hit one of these.

In the patch discussion, we were in agreement that it would be better
to have this knowledge up in lldb instead of depending on each
stub rewriting the pc behind the debugger's back, but that's a
larger scale change for another day.

<rdar://problem/65521634>
Differential revision: https://reviews.llvm.org/D91238