go/build: Set GOARCH on arm64 systems.

I am reliably informed that the architecture name and letter for the
plan9/inferno compilers for 64-bit ARM systems will be "arm64" and "7"
respectively, so let's get that bit in nice and early.

From Michael Hudson-Doyle.
https://codereview.appspot.com/34830045/


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@206374 138bc75d-0d04-0410-961f-82ee72b054a4