[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

llvm-svn: 242749