Use $GO_EXECUTABLE in Go-based lit tests

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254187 91177308-0d34-0410-b5e6-96231b3b80d8