bhyve: Add arm64 support to the gdb stub

• Add -G to the arm64 getopt handler.
• Add static register definitions and extensible XML definitions.
• Provide definitions for MD bits such as breakpoint encoding and length.
• Ensure that bhyve re-injects breakpoint exceptions that it is not responsible for.

Reviewed by: andrew
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D44740