go: Change from TARGET_ARCH to TUNE_PKGARCH

go: Change from TARGET_ARCH to TUNE_PKGARCH

Right now go-cross is changing signatures when you change TUNE for a given
architecture. In particular this breaks layer tests like:

yocto-check-layer ../meta-yocto-bsp/ --machines qemuarm beaglebone-yocto

This changes the PN addtion to something containing the tune rather than
the arch which avoids these kinds of errors. If go-cross can be tune
independent that would be nice but currently that isn't the case.

[YOCTO #12586]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>