When unwinding from the first frame, try to ask the remote debugserver

if this is a mapped/executable region of memory.  If it isn't, we've jumped
through a bad pointer and we know how to unwind the stack correctly based
on the ABI.  

Previously I had 0x0 special cased but if you jumped to 0x2 on x86_64 one
frame would be skipped because the unwinder would try using the x86_64 
ArchDefaultUnwindPlan which relied on the rbp.

Fixes <rdar://problem/10508291>

llvm-svn: 146477