tools cpupower bench: Override CFLAGS assignments

Allow user to specify outside CFLAGS values as make argument

Corrects an issue where CFLAGS is passed as a make argument for
cpupower, but bench's makefile does not inherit and append to them.

Signed-off-by: Stanley Chan <schan@cloudflare.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>