gcc-cross: Don't reference machine specific variables

gcc-cross: Don't reference machine specific variables

gcc-cross is installed into a package architecture specific directory
and is not meant to be machine specific. This patch replaces MACHINE_ARCH
with PACKAGE_ARCH to ensure this is really the case.

This was found by examining sstate checksums.

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