[lldb-server] Add setting to force 'g' packet use

Following up on https://reviews.llvm.org/D62221, this change introduces
the settings plugin.process.gdb-remote.use-g-packet-for-reading.  When
they are on, 'g' packets are used for reading registers.

Using 'g' packets can improve performance by reducing the number of
packets exchanged between client and server when a large number of
registers needs to be fetched.

Differential revision: https://reviews.llvm.org/D62931
diff --git a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.h b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.h
index 0e3e3b3..9ea3940 100644
--- a/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.h
+++ b/lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.h
@@ -284,6 +284,7 @@
   lldb::CommandObjectSP m_command_sp;
   int64_t m_breakpoint_pc_offset;
   lldb::tid_t m_initial_tid; // The initial thread ID, given by stub on attach
+  bool m_use_g_packet_for_reading;
 
   bool m_replay_mode;
   bool m_allow_flash_writes;