Fix appending -O0 to CFLAGS

The previous method would completely nerf CFLAGS once bsd.progs.mk had
recursed into the per-PROG logic and make the CFLAGS for tap testcases
to -O0, instead of appending to CFLAGS for all of the tap testcases.

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division