[cmake] pass GO_EXECUTABLE to llgo-go

[cmake] pass GO_EXECUTABLE to llgo-go

Summary:
When calling llgo-go from the llvm_add_go_executable
cmake function, specify $GO_EXECUTABLE as the go
command to call. Without this, llgo-go searches $PATH
which may be inconsistent with $GO_EXECUTABLE.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11290

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242749 91177308-0d34-0410-b5e6-96231b3b80d8