This commit adds the wasm architecture to the internal/bytealg package.

Some parts of the assembly code have been extracted from WebAssembly
bytecode generated with Emscripten (which uses musl libc).

Updates #18892

Change-Id: Iba7f7158356b816c9ad03ca9223903a41a024da6
Reviewed-on: https://go-review.googlesource.com/103915Reviewed-by: Keith Randall <khr@golang.org>