[libcxx] Set _LIBCPP_ELAST for mingw.

Reviewers: K-ballo, mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: jfb, jroelofs, majnemer, cfe-commits

Differential Revision: http://reviews.llvm.org/D6558

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@225273 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 656850f03e1ba912524c4ac6eab1a53777865950)

Change-Id: I97d6e6a2f4613f9f618cbec4ba7122699f572a47
4 files changed