goarch.bbclass: Replace logic for setting GOARM

goarch.bbclass: Replace logic for setting GOARM

The previous logic applied a regex to TUNE_FEATURES which could
set the GOARM value to 7 incorrectly, for example when dealing
with an arm1176 core. Simplify to check for the presence of
"armv7" instead. At the same time add a check for "armv6" and
set GOARM to 6 in that case.

Signed-off-by: Will Newton <willn@resin.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>