devel/gdb: Update to 12.1.

One notable feature included in 12.1 is async target support
permitting the use of commands like continue&.

In addition, this commit backports various post-12 commits to add
support for hardware breakpoints/watchpoints on aarch64, support for
resolving TLS variables from core dumps on amd64 and i386 via the
recently added NT_X86_SEGBASES core dump note, and support for
resolving TLS variables on arm and aarch64 via the recently added
NT_ARM_TLS register set.

Reviewed by:	pizzamig
Differential Revision:	https://reviews.freebsd.org/D35111