Casting a pointer to an int produces warnings with 64-bit targets.
Furthermore, an int is not always the optimal memory element that
can be copied in that case.
Let's use uintptr_t to cast pointers to integers for alignment
determination purposes, and mem_word_t to denote the optimal memory
"word" that can be copied on the platform.
The mem_word_t definition is equivalent to uintptr_t by default.
However, some 32-bit targets such as ARM platforms with the LDRD/STRD
instructions could benefit from word_t being an uint64_t.
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>