This package appears to include a private copy of valgrind.h. In the latest relase of valgrind 3.10.0 a bug was fixed for ppc32 and support was added for aarch64 and ppc64le. Please either update the private copy of valgrind.h (memcheck.h, callgrind.h, etc) or (better) make your package BuildRequires: valgrind-devel and use the system installed headers.
This was already fixed in http://pkgs.fedoraproject.org/cgit/mingw-glib2.git/commit/?id=3286bb7eaf13c82319a01d36b764ac85f1bb8f0a and the fix is included in upstream releases (which are also in Rawhide) from 2.41.2 onwards.
Note that isn't the actual upstream valgrind.h from 3.10.0 but a snapshot from svn. In particular it is missing ppc64le support. Could you double check that the final valgrind.h from valgrind 3.10.0 has really been incorporated?
I do not think that ppc64le support is too relevant to a MinGW package. ;-) The updated header from SVN was needed for a win64-specific fix. The upstream bug report to get valgrind.h updated for GLib is https://bugzilla.gnome.org/show_bug.cgi?id=736741 (which is more relevant to the glib2 package and bug 1141471, not this mingw package).