examples: Make libpng example sleep when showing error message on SCREEN
rtl-macro: Refactor macro adding code to allow macros to be filtered

With addIntrinsicMacros() split into hAddMacros() and hAddMacro(), we can
add checks to hAddMacros() to explicitly decide whether to add (or not to
add) each macro. This will be useful if, for example, individual macros
are target-specific.
Fix Hiword() for 64bit

Now there are two versions of Hiword(), one for 32bit and one for 64bit.

The 64bit version needs an extra operation to mask out the upper dword of
the 64bit Uinteger operand.

The 32bit version doesn't need that though, and I think it's best to keep
it separate from the 64bit version, in order to avoid the extra operation
which is unnecessary on 32bit and may not be optimized out.