hoard2: make block header size a multiple of 16 bytes

On 32bit x86 gcc11, allocations should be aligned to 16 bytes, but the
block header (which is stored before the allocations) contains two
pointers, which on a 32bit system results in 8 bytes. Add some padding
to make this header 16 bytes, guaranteeing that the data area is
actually aligned.

Fixes #18111.

Change-Id: Id23f8c0272d75090710f872b728852cd0fcb19ac
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5885
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

Switch back to hoard2 for now.

We'll reconsider rpmalloc when it has less overhead.