[WebAssembly] Fix incorrect DW_TAG_pointer_type size in test

It should be 64 bits not 32 for wasm64-unknown-unknown. I noticed this
because this test was failing in our CHERI fork of LLVM due to a larger
.debug_abbrev section size. It turns out this happens because we add
DW_AT_byte_size for non-default pointer sizes to allow debugging our
hybrid compilation mode where we can have two different kinds of pointers.

Reviewed By: aardappel

Differential Revision: https://reviews.llvm.org/D99410