go: fix on darwin, make go-1.7 build with clang-3.8. (#18001)

The darwin stdenv currently provides clang-3.7; however,

  a) go-1.7 currently expects a compiler that supports "-fdebug-prefix-map"
     arguments (which clang-3.8 supports but clang-3.7 does not) and

  b) even with clang-3.8, go-1.7 misinterprets the result of its runtime probes
     for support for the "-no-pie" flag, thereby failing to build runtime/cgo.

This patch resolves (a) by building go-1.7 with clang-3.8 and resolves (b) by
teaching go how to correctly probe "-no-pie" error messages from clang.