Checking _64BIT_ flags, adding ARMFIX

Checking all _64BIT_ flags and adding relevant ARMFIX comments around to
make sure we fix them at the same time. There are a number of different
types: ARMFIX (generic), ARMFIX-HEAP-SIZE / ARMFIX-STACK-SIZE (related
to assuming the stack size can be larger on 64-bit - mostly harmless),
ARMFIX-PACK (pack push/pop, probably harmless).

Some of the generic ones have comments as to what to do, others are
just enquires to learn more about what that meant and why is it different
for 64-bit architectures.