cmd/mem.c: fix wrong use of ifdef, drop pointless SUPPORT_64BIT_DATA macro

The macro MEM_SUPPORT_64BIT_DATA is always defined, as either 1 or 0,
so using "#ifdef MEM_SUPPORT_64BIT_DATA" doesn't do what one
expects.

This means that currently all 32 bit targets get compiled with the .q
suffix mentioned in the help text, while it doesn't actually work.

Use the proper "#if" instead.

There's really no point defining another similarly-named macro with
exactly the same value, so just use MEM_SUPPORT_64BIT_DATA throughout.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>