bhyve: Prepare to add arm64 support to the gdb stub

In particular:

• Stop assuming that the breakpoint size is one byte.
• Avoid referencing the "rip" field in machine-independent code, use a helper.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D43483