multilib: Fix an OVERRIDES expansion order issue

There were problems where a SRC_URI with:

SRC_URI_append_powerpc = " xxx"
SRC_URI_append_powerpc64 = " xxx2"

would end up with *both* xxx and xxx2 being added when using a multilib
which is clearly incorrect and undesirable.

The issue is that OVERRIDES has virtclass-multilib-xxxx added to it,
this eventually changed DEFAULTTUNE which then changes
TRANSLATED_TARGET_ARCH which is in OVERRIDES meaning we then need to
re-evaluate the overides and the TRANSLATED_TARGET_ARCH gets applied
twice since once you apply an override, it doesn't get undone.

Expanding DEFAULTTUNE to the correct value in advance avoids the issue
and means only the correct overrides get applied.

[YOCTO #3874]

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