sstate: do not add TARGET_ARCH to pkgarch for cross recipes.

sstate: do not add TARGET_ARCH to pkgarch for cross recipes.

This is redundant (target arch is already in PN), and breaks
compiling a cross-canadian toolchain, as that needs populating the
sysroot with two different native-hosted toolchains built from
cross recipes. Inserting TARGET_ARCH allows only one or the other.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>