classes/nativesdk: also override TUNE_PKGARCH

classes/nativesdk: also override TUNE_PKGARCH

The nativesdk class overrides PACKAGE_ARCH and unsets TUNE_FEATURES, but
as recipes might want to look at TUNE_PKGARCH too (for example, when
setting QEMU_EXTRAOPTIONS) we should also override that variable.

Otherwise, a nativesdk recipe will have the TUNE_PKGARCH of the target,
which leads to errors (eg passing mips arguments to an arm qemu).

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>