Add gdb-remote P register write test.

Added test that attempts to write a value to each general
purpose register that is bit-flipped from the initial read
value.  It then reads it back to see if it takes.

Right now I just assert that at least one register bit flip
write succeeds.  I added a note that on the MacOSX x86_64
debugserver case, the only writes that succeed from the GPR
set are rax thru rdx, rdi, rsi and rbp.  The failures are E32
failure-at-write-attempt issues on the debugserver end.
I'll revisit this after implementing in the llgs linux-x86_64 branch.
The packet log looks good but I may have a subtle mistake in the code.

llvm-svn: 210681