Export public functions implemented in assembly on Windows.

Summary:
By default, symbols aren't visible outside of the module that defines
them. To make them visible, they must be exported. The easiest way to do
that is to embed an `-export:symname` directive into the object file.

Reviewers: mstorsjo, rnk

Subscribers: christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D51508

llvm-svn: 341232