gdb: xtensa-linux: add call0 support

gdb: xtensa-linux: add call0 support

Correctly handle a0- registers. This allows debugging call0 code in
linux natively.
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/
2017-03-27  Max Filippov  <jcmvbkbc@gmail.com>

	* xtensa-linux-nat.c (fill_gregset): Call regcache_raw_collect
	for a single specified register or for all registers in
	a0_base..a0_base + C0_NREGS range.
	(supply_gregset_reg): Call regcache_raw_supply for a single
	specified register or for all registers in a0_base..a0_base +
	C0_NREGS range.