Add a new wasm32-wasip1 target to rustc

[alexcrichton]

This commit adds a new target called wasm32-wasip1 to rustc. This new target is explained in these two MCPs:

• Rename wasm32-wasi target to wasm32-wasi-preview1 compiler-team#607
• Smooth the renaming transition of wasm32-wasi compiler-team#695

In short, the previous wasm32-wasi target is going to be renamed to wasm32-wasip1 to better live alongside the new wasm32-wasip2 target. This new target is added alongside the wasm32-wasi target and has the exact same definition as the previous target. This PR is effectively a rename of wasm32-wasi to wasm32-wasip1. Note, however, that as explained in rust-lang/compiler-team#695 the previous wasm32-wasi target is not being removed at this time. This change will reach stable Rust before even a warning about the rename will be printed. At this time this change is just the start where a new target is introduced and users can start migrating if they support only Nightly for example.