[aarch64-darwin] be very careful of warnings.

So we did *not* have the stgCallocBytes prototype, and subsequently
the C compiler defaulted to `int` as a return value. Thus generating
sxtw instructions for the return value of stgCalloBytes to produce
the expected void *.

rts/linker/macho/plt.c

 #include "Rts.h"
 #include "plt.h"
+#include "RtsUtils.h"
 
 #if defined(aarch64_HOST_ARCH)