tsan: allow to cross-build Go runtime

Allow user to provide target GOOS/GOARCH.
If not provided, use the host GOOS/GOARCH as we do now.
This allows to cross-compile the runtime.
Also provide SKIP_TEST knob for cross-compilation
since the test will most likely not run on host.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D150650