Add specs for the x64-mingw32 platform.
Add proper support for x64-mingw32 platform.

This fixes github issue #2356 .
Add more variants to specs for GemHelpers#generic
Remove useless code from GemHelpers#generic
Merge pull request #2590 from larskanis/add_x64-mingw32_support

Add support for the x64-mingw32 platform