cmd/go: pass -arch to GOOS=darwin host compiler, linker

[rsc]

See golang/vscode-go#1087.
The root cause is confusion about the Mac toolchain expecting x86 vs arm binaries.
We should be able to fix the confusion by forcing when GOOS=darwin the use of -arch arm64 or -arch x86_64 (not sure the second is spelled correctly), to match GOARCH.

This would mean adding the flags in the C compiler invocation and also adding them to the -extld passed to the Go linker.

[bcmills]

I think this is a duplicate of #43476, or at least very closely related.

[bcmills]

If I understand https://clang.llvm.org/docs/CrossCompilation.html correctly, we might want to pass the -arch and/or -target flags to clang on all GOOSes (at least when cross-compiling), not just darwin.

I suppose darwin is somewhat unique in that apparently we can't easily determine whether clang thinks it is cross-compiling.

[rsc]

At this point in the release cycle I'd be much more comfortable only doing it for darwin.
This may be a duplicate of #43476 but I think this bug states the problem more clearly if nothing else.

[gopherbot]

Change https://golang.org/cl/283812 mentions this issue: cmd/dist, cmd/go: pass -arch for C compilation on Darwin by default