gdb: xtensa-linux: support THREADPTR register

gdb: xtensa-linux: support THREADPTR register

Make THREADPTR user register accessible.

gdb/
2017-03-27  Max Filippov  <jcmvbkbc@gmail.com>

	* xtensa-linux-nat.c (fill_gregset): Call regcache_raw_collect
	for THREADPTR register.
	(supply_gregset_reg): Call regcache_raw_supply for THREADPTR
	register.
	* xtensa-tdep.c (XTENSA_DBREGN_UREG): New definition.
	(xtensa_derive_tdep): Initialize tdep->threadptr_regnum.
	* xtensa-tdep.h (gdbarch_tdep::threadptr_regnum): New field.