tools/ocaml/libs: Allocate the correct amount of memory for Abstract_tag

tools/ocaml/libs: Allocate the correct amount of memory for Abstract_tag

caml_alloc() takes units of Wsize (word size), not bytes.  As a consequence,
we're allocating 4 or 8 times too much memory.

Ocaml has a helper, Wsize_bsize(), but it truncates cases which aren't an
exact multiple.  Use a BUILD_BUG_ON() to cover the potential for truncation,
as there's no rounding-up form of the helper.

Fixes: 8b7ce06a2d34 ("ocaml: Add XC bindings.")
Fixes: d3e649277a13 ("ocaml: add mmap bindings implementation.")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit 36eb2de31b6ecb8787698fb1a701bd708c8971b2)