When UBSan is enabled, I noticed runtime errors complaining of shifting
of negative numbers.

This patch fixes this by reusing existing macros from the ARM port.

It also removes unused macros from AArch64's port.

gdb/ChangeLog:

2020-12-04  Luis Machado  <luis.machado@linaro.org>

	* aarch64-tdep.c (submask, bit, bits): Remove.
	* arch/aarch64-insn.c (extract_signed_bitfield): Remove.
	(aarch64_decode_adr, aarch64_decode_b aarch64_decode_bcond)
	(aarch64_decode_cb, aarch64_decode_tb)
	(aarch64_decode_ldr_literal): Use sbits to extract a signed
	immediate.
	* arch/aarch64-insn.h (submask, bits, bit, sbits): New macros.