gcc-multilib: Simply/fix MULTILIB_OPTIONS handling

gcc-multilib: Simply/fix MULTILIB_OPTIONS handling

MULTILIB_OPTIONS takes the parameters which trigger a given multilib to be
selected. It supports *one* option per multilib, '/' separated. Spaces
separate options used to generate additional multilib combinations.

Adding in all of CFLAGS to this is therefore clearly a really bad idea
but how do we fix things?

The best option I've come up with so far is a list of whitelist variables
to use to trigger the multilibs. Its populated with the standard multilibs
we support, anyone setting up an advanced multilib can populate the variable
with the correct trigger parameters.

This has the advantage of simplifying the code and allowing us to remove
the code filtering blocks since there is no longer option duplication. Testing
after this change shows a much improved sdk toolchain functionality.

(From OE-Core rev: 29202cd1b9d2e5d56e5b9f7a596e44e229c90492)

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