gdb_server: support qXfer:threads:read packet

gdb_server: support qXfer:threads:read packet

This patch adds support for the qXfer:threads:read packet. In addition
to providing a more efficient method of updating thread state, recent
versions of GDB (7.11.1 and up) can also report remote thread names.
While thread names are not enabled in this patch due to its limited
applicability at the moment, it can be enabled at a later date with
little effort.

As a part of revamping how threads are presented to GDB, extra info
strings for each of the supported RTOSes were updated to match
conventions present in the GDB source code. For more information, see
remote_threads_extra_info() in remote.c. This results in a much smoother
experience when interacting with GDB.

It is also worth mentioning that use of qXfer:threads:read works around
a number of regressions in older versions of GDB regarding remote thread
display. Trust me, it's great.

Change-Id: I97dd6a93c342ceb9b9d0023b6359db0e5604c6e6
Signed-off-by: Steven Stallion <stallion@squareup.com>
Reviewed-on: http://openocd.zylin.com/3559
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>