[WebAssembly] Change wasm SizeType to match asmjs

Summary:
We want wasm and asmjs to have matching ABIs, and right now asmjs uses
unsigned int for its size_t. This causes exported symbols in libcxx to
not match and can cause weird breakage where libcxx doesn't get linked
as a result.  Long-term we probably want wasm32, wasm64, and asmjs to
all use unsigned long, but that would cause unnecessary ABI churn for
asmjs so defer that until we can make all the ABI changes at once.

Patch by Jacob Gravelle

Differential Revision: https://reviews.llvm.org/D24134

llvm-svn: 280420