Support compiling on MSYS2

[qmfrederik]

This patch consists of two commits:

• The first commit contains a minimal set of changes to make libdispatch compile on the MSYS2 environments. There are slight differences in headers, which are accounted for, and the -fms-extensions variable is passed to clang to enable the __popcnt64 intrinsic
• The second commit adds a GitHub actions job to validate the changes

If this project doesn't want to use GitHub actions, I can remove the second commit.

[qmfrederik]

@MaxDesiatov Roger that, removed the GitHub actions.

[qmfrederik]

@compnerd I've used CMake checks for pid_t and mode_t, and refined the usage of -fms-extensions. Let me know if this helps.

[compnerd]

@swift-ci please test

[qmfrederik]

OK - looks like CI for Windows passed: https://ci-external.swift.org/job/swift-corelibs-libdispatch-PR-windows/55/consoleFull

[MaxDesiatov]

@swift-ci test windows