Allow the kernel to build with a compiler that sets -fno-common.

The mechanism that generates assym.inc and offset.inc depends on the
symbols in question being common. For now, simply force the object files
to be created with -fcommon.

-fno-common will be the default in GCC10/LLVM11.

Submitted by: arichardson
Reviewed by: kevans
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24322