native/nativesdk: Clear MACHINEOVERRIDES

native/nativesdk: Clear MACHINEOVERRIDES

In cases where we override variables e.g PACKAGE_ARCH
conditionally and use a MACHINEOVERRIDE to qualify the
override like

PACKAGE_ARCH_<MACHINE> = "some target PACKAGE_ARCH"

This would also be effective if this recipe is
BBEXTENDED to native or nativesdk which is undesired
This particular issue will cause wrong sysroot to be
used for nativesdk case since PACKAGE_ARCH would have
been modified and eventually changing value of
STAGING_DIR_TARGET which we use as --sysroot option
to cross and cross-canadian built SDK gcc

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>