sstate.bbclass: fix multimachine builds when PACKAGE_ARCH = MACHINE_ARCH

sstate.bbclass: fix multimachine builds when PACKAGE_ARCH = MACHINE_ARCH

previously you could only build one set of packages for multiple machines:

MACHINE=foo bitbake task-base
MACHINE=bar bitbake task-base

would only create task-base packages for foo, but not for both foo and bar.
Doing MACHINE=bar bitbake task-base -c cleanall would remove the packages for foo.

The solution is to use MULTIMACH_ARCH as suggested by Richard Purdie.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Saul Wold <sgw@linux.intel.com>