reduce CodeCapacity value to 16MB in bootimage.cpp

32MB was just slightly too large for PowerPC immediate call instructions
to span, and 16MB matches the JIT executable memory area we use in
compile.cpp.

src/bootimage.cpp

@@ -389,7 +389,7 @@ main(int ac, const char** av)
   Processor* p = makeProcessor(s, h, false);
 
   BootImage image;
-  const unsigned CodeCapacity = 32 * 1024 * 1024;
+  const unsigned CodeCapacity = 16 * 1024 * 1024;
   uint8_t* code = static_cast<uint8_t*>(h->allocate(CodeCapacity));
   p->initialize(&image, code, CodeCapacity);