gcc-target: Ensure buildtools-extended-tarball doesn't use arch=native

A nativesdk BBCLASSEXTEND was added to gcc-target without realising this
would pass arch=native through to it for x86-64. This heavily optimises
gcc output for the host its running on meaning it can't be reused via
sstate on other machines.

Add class-target overrides here to get the desired behaviour. All
targets have been covered for completeness.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>