Use nproc and sysctl -n hw.cpu for default initializing CORES

This avoids having to pass --build-arg CORES=n when building the
docker images.

build-mingw-w64.sh

@@ -23,6 +23,8 @@ else
     true
 fi
 
+: ${CORES:=$(nproc 2>/dev/null)}
+: ${CORES:=$(sysctl -n hw.ncpu 2>/dev/null)}
 : ${CORES:=4}
 : ${ARCHS:=${TOOLCHAIN_ARCHS-i686 x86_64 armv7 aarch64}}