Discover support of 'p' packet.

Some stubs only support g/G packets for registers.
This change makes sure that we check if remote stub supports 'p' packet before using it.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189576 91177308-0d34-0410-b5e6-96231b3b80d8