userland build: replace -fno-common with ${CFCOMMONFLAG}

This change allows any downstream or otherwise consumer to easily override
the new -fno-common default on a temporary basis without having to hack into
src.sys.mk, and also makes it a bit easier to search for these specific
cases where -fno-common must be overridden with -fcommon or else the build
will fail.

The gdb build, the only program requiring -fcommon on head/, is switched
over as an example usage. It will need it on all branches, so this does not
harm future mergability.

MFC after: 3 days