kbuild: Disable gcc plugins

The GCC plugin feature leads to different kernel configurations on what
ought to be equivalent build systems because they depend on the build
hosts native compilers rather than the cross compilers needed for the
target. This causes problems with module symbol version mismatches.

Disable GCC plugins for all build hosts.

Advanced build script hackery borrowed from a patch by milhouse.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>