[Driver] Automatically enable -munwind-tables if -fseh-exceptions is enabled

For targets where SEH exceptions are used by default (on MinGW,
only x86_64 so far), -munwind-tables are added automatically. If
-fseh-exeptions is enabled on a target where SEH exeptions are
availble but not enabled by default yet (aarch64), we need to
pass -munwind-tables if -fseh-exceptions was specified.

Differential Revision: https://reviews.llvm.org/D55749

llvm-svn: 349452