[Targets] Move addCygMingDefines into the arch-independent Targets.cpp (NFC)

This fixes a dependency inconsistency, where addMinGWDefines in Targets.cpp
(used from other architectures than X86) called the addCygMingDefines function
in X86.h.

This was inconsistently split in SVN r308791 (D35701).

Differential Revision: https://reviews.llvm.org/D36072

llvm-svn: 309598