EXPORTED_SYMBOL_FILE using mingw and cmake

Summary: This is a fix for the command line syntax error while building LTO when using MinGW.

Patch By: jsroemer

Reviewers: rnk

Reviewed By: rnk

Subscribers: rnk, beanz, llvm-commits

Differential Revision: http://reviews.llvm.org/D5476

llvm-svn: 220935