Revert "gdb: Do not print empty-group regs when printing general ones"

Revert "gdb: Do not print empty-group regs when printing general ones"

Revert the change since it breaks existing behavior of "info registers"
for some architectures. At least AArch64 and ARM are impacted by this change.

gdb/ChangeLog:

2020-03-04  Luis Machado  <luis.machado@linaro.org>

	Revert aa66aac47b4dd38f9524ddb5546c08cc09930d37 due to regressions
	in "info registers" for AArch64/ARM.

	The change caused "info registers" to not print GPR's.

	gdb/ChangeLog:

	2020-02-01  Shahab Vahedi  <shahab@synopsys.com>

	* target-descriptions.c (tdesc_register_in_reggroup_p): Return 0
	when reg->group is empty and reggroup is not.