gcc-common: Only apply fpu settings to target gcc

Within the OE build environment, we supply the correct fpu settings. These
only need to be spelt out for the on-target gcc.

Doing this means the checksums for the core compiler don't depend on the fpu
settings. We exclude the compiler tunes for similar reasons, it doesn't need
to influence the compiler build.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>