Add -fseh-exceptions for MinGW-w64

This adds a flag called -fseh-exceptions that uses the native Windows
.pdata and .xdata unwind mechanism to throw exceptions. The other EH
possibilities are DWARF and SJLJ exceptions.

Patch by Martell Malone!

Reviewed By: asl, rnk

Differential Revision: http://reviews.llvm.org/D3419