[llvm-go] parameterize $GOPATH construction

Summary:
To build llgo, you must currently ensure that llgo
is in the tools/llgo directory, due to a hard-coded
path in llvm-go.

To support the use of LLVM_EXTERNAL_LLGO_SOURCE_DIR,
we introduce a flag to llvm-go that enables the
caller to specify the paths to symlink in the
temporary $GOPATH.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D21634

llvm-svn: 276829