gdb: xtensa: fix test for privileged register number

gdb: xtensa: fix test for privileged register number

There are no a0-a15 pseudoregisters at the top of the register set in
call0 registers layout. All registers above gdbarch_num_regs (gdbarch)
are privileged. Treat them as such. This fixes the following gdb
assertion seen when 'finish' command is invoked:

  regcache.c:649: internal-error: register_status
  regcache_raw_read(regcache*, int, gdb_byte*):
  Assertion `regnum >= 0 && regnum < regcache->descr->nr_raw_registers'
  failed.

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

	* xtensa-tdep.c (xtensa_pseudo_register_read): Treat all
	registers above gdbarch_num_regs (gdbarch) as privileged in
	call0 ABI.