[llgo] Force exporting __morestack from llgoi

Summary:
Force exporting __morestack from llgoi, so that
the symbol is available to the execution engine
when linking with libLLVM.so. The engine does
not reference __morestack explicitly, so must
be provided by the host program.

Reviewers: pcc

Subscribers: axw, llvm-commits

Differential Revision: http://reviews.llvm.org/D12625

llvm-svn: 254188