Add "QAddressIsExecutable" packet to debugserver. Used to test

whether a given address is in an executable region of memory or
not.  I haven't written the lldb side that will use this packet it
hasn't been tested yet but it's a simple enough bit of code.

I want to have this feature available for the unwinder code.  When
we're stopped at an address with no valid symbol context, there are
a number of questions I'd like to ask --

  is the current pc value in an executable region (e.g. did they
  jump to unallocated/unexecutable memory?  we know how to unwind
  from here if so.)

  Is the stack pointer or the frame pointer the correct register
  to use to find the caller's saved pc value?

Once we're past the first frame we can trust things like eh_frame
and ABI unwind schemes but the first frame is challenging and having
a way to check potential addresses to see if they're executable or
not would help narrow down the possibilities a lot.

llvm-svn: 144074