[WebAssembly] Rename relocations from R_WEBASSEMBLY_ to R_WASM_

See https://github.com/WebAssembly/tool-conventions/pull/95.

This is less typing and IMHO more readable, and it also fits with
our naming around the binary format which tends to use the short name.
e.g.

include/llvm/BinaryFormat/Wasm.h
tools/llvm-objdump/WasmDump.cpp
etc..

Differential Revision: https://reviews.llvm.org/D57611

llvm-svn: 353062