Revert to msvcrt.dll as LIBC on MINGW

ffi-1.13.0 switched FFI::Library::LIBC from msvcrt.dll to ucrtbase.dll
as part of ffi#779 in commit c674683 .

As described in ffi#788 ucrtbase.dll has behavioural changes which
shouldn't be released as part of a minor version change of ffi.
While the change makes sense for mswin, we revert it for mingw.

Fixes ffi#788

lib/ffi/platform.rb

@@ -129,7 +129,11 @@ def self.is_os(os)
     end
 
     LIBC = if IS_WINDOWS
-      "ucrtbase.dll"
+      if RbConfig::CONFIG['host_os'] =~ /mingw/i
+        RbConfig::CONFIG['RUBY_SO_NAME'].split('-')[-2] + '.dll'
+      else
+        "ucrtbase.dll"
+      end
     elsif IS_GNU
       GNU_LIBC
     elsif OS == 'cygwin'