Really big cleanup.

Really big cleanup.
- New target type "mingw" was introduced
- Same things for both mingw & cygwin are marked as "cygming" (as in
gcc)
- .lcomm is supported here, so allow LLVM to use it
- Correctly use underscored versions of setjmp & _longjmp for both mingw
& cygwin

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32833 91177308-0d34-0410-b5e6-96231b3b80d8