Fix shifting of negative value
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 <email@example.com> * 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.