Add new tests dbregs_dr[0123]_trap_code in arch/amd64

Add new tests:
- dbregs_dr0_trap_code
- dbregs_dr1_trap_code
- dbregs_dr2_trap_code
- dbregs_dr3_trap_code

This is the final set of tests for trap types that are supposed to be
supported by all amd64 CPUs.

Traps for code (instruction) execution must be set to 1 byte, otherwise
they are undefined. x86 code traps must point to the first byte of an
instruction.

Sponsored by <The NetBSD Foundation>

(kamil)