Add libwinpthread back to core_dll_defs

[ntkme]

ruby/date#126 (comment)

[ntkme]

@larskanis I think the yaml failure on head-x64 is caused by head-x64 package being a week ago that it is probably built with mingw-w64 <=r688, however, when this repo is packing ri2, it's always using latest mingw-w64 (r724 as of now). Such mismatch will cause problem. You can see head-arm works just fine. I think if you can get a new head-x64 build everything should be good after that.

[larskanis]

@ntkme Yes, you're right. I had some older artifacts in the build directory. Closing this PR since it is part of 6e29749 .