Use $GO_EXECUTABLE in Go-based lit tests

Summary:
When running tests, pass the GO_EXECUTABLE CMake
cache variable to llvm-go. The "go" binary may
not be in $PATH, or may be different to the one
passed to CMake.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14041

llvm-svn: 254187