Use $GO_EXECUTABLE in Go-based lit tests

[axw]

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.

[pcc]

LGTM

test/lit.cfg

def find_tool_substitution(pattern):
     # llvm-lit "-Dllc=llc -enable-misched -verify-machineinstrs"
     tool_path = lit_config.params.get(tool_name)
     if tool_path is None:
         tool_path = lit.util.which(tool_name, llvm_tools_dir)
     if (tool_name == "llc" and
        'LLVM_ENABLE_MACHINE_VERIFIER' in os.environ and
        os.environ['LLVM_ENABLE_MACHINE_VERIFIER'] == "1"):
         tool_path += " -verify-machineinstrs"
+    if (tool_name == "llvm-go"):
+        tool_path += " go=" + config.go_executable
     return tool_name, tool_path, tool_pipe
 
 
 for pattern in [r"\bbugpoint\b(?!-)",
                 NOJUNK + r"\bllc\b",
                 r"\blli\b",
                 r"\bllvm-ar\b",
                 r"\bllvm-as\b",