build: stop providing an explicit -fno-common

[kevans]

Summary

All of the compilers used to build FreeBSD should now default to
-fno-common, so this isn't really necessary anymore -- it was only
added to avoid regressing the -fno-common build before it became the
default everywhere.

For <kmod.mk>, we'll leave that one in place. I didn't actually add
that one, and it's more important that we always have -fno-common in
kmod builds.

[emaste]

GCC's change, for reference: https://gcc.gnu.org/cgit/gcc/commit/?id=6271dd984d7f92

sys/conf/kmod.mk

151

why not this one?

[brooks]

Unless I've missed a back out, the LLVM commit appears to have been included in LLVM 11 which is older than we care about https://github.com/llvm/llvm-project/commit/0a9fc9233e172601e26381810d093e02ef410f65