[AArch64] [MinGW] Allow enabling SEH exceptions

The default still is dwarf, but SEH exceptions can now be enabled
optionally for the MinGW target.

Differential Revision: https://reviews.llvm.org/D55748

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349451 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed