Read the breakpoint instruction to determine its length in BKPT_SKIP.

This fixes continuing from debug.kdb.enter=1 after enabling the use of
compressed instructions since the compiler can emit the two byte
c.ebreak instead of the 4 byte ebreak.

Reviewed by: br
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D23507