gdbserver: xtensa: add call0 support

gdbserver: xtensa: add call0 support

Correctly handle a0- registers on requests from remote gdb. This fixes

  'Register 1 is not available'

and subsequent assertion in the remote gdb connecting to the gdbserver:

  'findvar.c:291: internal-error: value_of_register_lazy:
    Assertion `frame_id_p(get_frame_id (frame))' failed.'

The register structure is the same for windowed and call0 ABIs because
currently linux kernel internally requires windowed registers, so they
are always present.

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

	* linux-xtensa-low.c (xtensa_fill_gregset): Call collect_register
	for all registers in a0_regnum..a0_regnum + C0_NREGS range.
	(xtensa_store_gregset): Call supply_register for all registers in
	a0_regnum..a0_regnum + C0_NREGS range.