Disable 'C'-compressed ISA extension.

It works excellent, but KDB disassembler and DTrace FBT provider for
RISC-V do lack support for it. They currently handle 4-byte instructions
only, while C-compressed ISA extension introduces 2-byte instructions
freely mixing them together.

So disable it for now.

Reviewed by: markj@
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16436