nativesdk-buildtools-perl-dummy: properly set PACKAGE_ARCH

nativesdk-buildtools-perl-dummy: properly set PACKAGE_ARCH

Turns out I did a silly thing in OE-Core revision
9b1831cf4a2940dca1d23f14dff460ff5a50a520 and forgot to remove the
explicit setting of PACKAGE_ARCH outside of the anonymous python
function; the original bug was apparently fixed but the functionality of
allarch.bbclass was being disabled because it was able to see that
PACKAGE_ARCH was not set to "all" - which was what I was trying to
ensure.

(From OE-Core rev: a25ab5449825315d4f51b31a634fe6cd8f908526)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>