[WebAssembly] Change size_t to `unsigned long`.

Changing it to unsigned long (which is 32-bit on wasm32) makes it the same
type as wasm64 (where unsigned long is 64-bit), which would eliminate the most
common cause for mangled names being different between wasm32 and wasm64. For
example, export lists containing symbol names could now often be the same
between wasm32 and wasm64.

Differential Revision: https://reviews.llvm.org/D40526

llvm-svn: 337783