Redirect strftime_l to the locale-ignorant strftime on mingw

_strftime_l is only available in the numbered msvcrt versions
(starting from msvcr80.dll). In the default configuration, mingw
targets the unversioned msvcrt.dll - and there, _strftime_l is
not available (not even on windows 10).

If __MSVCRT_VERSION__ is set to a higher value (indicating a
non-default target and wanting to link to msvcrXX.dll), use the
correct function.

Differential Revision: https://reviews.llvm.org/D37468

llvm-svn: 312617