bhyve: Simplify register definitions a bit

It's awkward to have separate tables for information which is logically
connected. Merge the gdb_regset[] and gdb_regsize[] arrays and update
gdb_read_regs() to cope with the result. This makes the addition of
arm64 support a bit cleaner.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D43481