mempool: make alignment/rounding 64-bit compatible

Minimum alignment and rounding must be done on a word boundary. Let's
replace _ALIGN4() with WB_UP() which is equivalent on 32-bit targets,
and 64-bit aware.

Also enforce a minimal alignment on the memory pool. This is making
a difference mostly on64-bit targets where the widely used 4-byte
alignment is not sufficient.

The _ALIGN4() macro has no users left so it is removed.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>