xtensa: implement NPTL helpers

These changes allow debugging multithreaded NPTL xtensa applications.

2015-08-20  Max Filippov  <jcmvbkbc@gmail.com>
gdb/gdbserver/
	* configure.srv (xtensa*-*-linux*): Add srv_linux_thread_db=yes.
	* linux-xtensa-low.c (arch/xtensa.h gdb_proc_service.h): New
	#includes.
	(ps_get_thread_area): New function.

2015-08-20  Max Filippov  <jcmvbkbc@gmail.com>
gdb/
	* arch/xtensa.h: New file.
	* xtensa-linux-nat.c (gdb_proc_service.h): New #include.
	(ps_get_thread_area): New function.
	* xtensa-linux-tdep.c (xtensa_linux_init_abi): Add call to
	set_gdbarch_fetch_tls_load_module_address to enable TLS support.
	* xtensa-tdep.c (osabi.h): New #include.
	(xtensa_gdbarch_init): Call gdbarch_init_osabi to register
	xtensa-specific hooks.
	* xtensa-tdep.h (struct xtensa_elf_gregset_t): Add threadptr
	member and move the structure to arch/xtensa.h.