[llgo] update libgo-llgo.so version to match libtool_VERSION

Summary:
libtool_VERSION was changed in gofrontend a while ago,
but CMakeLists.txt in llgo wasn't updated, and so the
install target fails. Not sure how this went unnoticed
for so long.

Reviewers: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11118

llvm-svn: 242291