Enable 'C'-compressed ISA extension.

This was disabled recently due to lack of support in KDB disassembler
and DTrace FBT provider. Support for 'C'-extension to both of these was
added, so we can now enable 'C'-extension.

This reduces size of the kernel important for low-end embedded devices,
and saves cache footprint for high perfomance machines.

Approved by: re (kib)
Sponsored by: DARPA, AFRL