cross: Update MULTIMACH_TARGET_SYS to reflect modern recipes
Mixing PACKAGE_ARCH with BUILD_* has always seemed somewhat odd. This
comes into play when building different SDKMACHINES as the PN space
for the recipes did once overlap. With the modern build process we
use, this overlap is a thing of the past since PN has SDK_SYS appended
to it.
cross.bbclass sets PACKAGE_ARCH == BUILD_ARCH so this change only affects
crosssdk.bbclass where PACKAGE_ARCH == SDK_ARCH.
The current structure is quite confusing and this removes this artefact
of history, allowing other gcc cleanups rather than meaning we futher
complicate the code.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
