sys: Switch the kernel's C standard from C99 to GNU99.

This matches the default used in userland, and the kernel already
depends on various GNU extensions to standard C that are supported by
both clang and GCC.

This should be a no-op for clang, but for GCC it enables some GNU
extensions that aren't otherwise enabled. It also enables GCC for
i386 to avoid the need for a floatundidf intrinsic in libkern.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40646