mem-pool: don't assume uintmax_t is aligned enough for all types

mem-pool: don't assume uintmax_t is aligned enough for all types

mem_pool_alloc uses sizeof(uintmax_t) as a proxy for what should be
_Alignof(max_align_t) in C11. On most architectures this is sufficient
(though on m68k it is in fact overly strict, since the de-facto ABI,
which differs from the specified System V ABI, has the maximum alignment
of all types as 2 bytes), but on CHERI, and thus Arm's Morello
prototype, it is insufficient for any type that stores a pointer, which
must be aligned to 128 bits (on 64-bit architectures extended with
CHERI), whilst uintmax_t is a 64-bit integer.

Fix this by introducing our own approximation for max_align_t and a
means to compute _Alignof it without relying on C11. Currently this
union only contains uintmax_t and void *, but more types can be added as
needed.

Signed-off-by: Jessica Clarke <jrtc27@jrtc27.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>