go-cross: take GOARM environment setting

go-cross: take GOARM environment setting

Instead of hard-coding GOARM to ${TARGET_GOARM} in
the wrapper script, take it from an existing
environment setting if present.  This allows the
same cross-compiler to be used for different ARM
targets.

Signed-off-by: Matt Madison <matt@madison.systems>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>