goarch.bbclass: do not leak TUNE_FEATURES into crosssdk task signatures

goarch.bbclass: do not leak TUNE_FEATURES into crosssdk task signatures

The default assignments look like this:
TARGET_GO386 = "${@go_map_386(d.getVar('TARGET_ARCH'), d.getVar('TUNE_FEATURES'), d)}"

TUNE_FEATURES is a target-specific variable, and so should be used
only for target builds. The change is similar to what is already done
for native packages.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>