Added gdb-remote P/p thread suffix test.

Improved the P writes all GPR register values test.  It now
limits itself to GPR registers that are not containers for
other registers.  Pulled in improvements from the llgs branch.

Note on Linux llgs I'm able to write a much wider range
of registers successfully with $P using the bitflip test than I am able
to write with debugserver.  Might be worth drilling into.

llvm-svn: 210818