Update mingw-w64 to the latest version

This version fixes deadlocks in winpthreads condvars (in certain
use patterns), uses the _UCRT define to request and signal the
use of UCRT, fixes the UCRT _vsnwprintf function, uses the
CRT's setjmp/longjmp on AArch64 if SEH is in use, and makes the
UCRT set the app type to console properly for such apps.

build-mingw-w64.sh

@@ -53,7 +53,7 @@ cd mingw-w64
 
 if [ -n "$SYNC" ] || [ -n "$CHECKOUT" ]; then
     [ -z "$SYNC" ] || git fetch
-    git checkout 164990461720e0ed6ea5ea9f359d78595b1a459a
+    git checkout 19e51f596a126aa5019c126e6904efca771493d9
 fi
 
 # If crosscompiling the toolchain itself, we already have a mingw-w64