toolchain/gcc/clang: Update PREFERRED_PROVIDER to work with multilibs

toolchain/gcc/clang: Update PREFERRED_PROVIDER to work with multilibs

As currently written, the PREFERRED_PROVIDER entries don't work with multilib
since they are added after the manipulations by multilib_global at ConfigParsed
time. We therefore need to spell out the configs correctly for multilib.

To avoid variable overwritten warnings from the expansion, we then have to
expand out the nativesdk providers usng class overrides.

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