Rollup merge of #148969 - Zalathar:wasm32-uwu, r=jieyouxu

compiletest: Don't apply "emscripten" directives to `wasm32-unknown-unknown`

This special case dates all the way back to the original introduction of the `wasm32-unknown-unknown` target, in rust-lang/rust#45905.

Either it isn't needed, in which case we should remove it, or it *is* needed, in which case we should fix the directives in any affected tests.

Note that while the intent of this code was presumably to make `//@ ignore-emscripten` also ignore tests on w32-u-u, it has the unfortunate side-effect of also causing `//@ only-emscripten` tests to *run* on w32-u-u, which is potentially very confusing.

Author: Zalathar

src/tests/directives.md

@@ -141,8 +141,7 @@ Some examples of `X` in `ignore-X` or `only-X`:
 - OS: `android`, `emscripten`, `freebsd`, `ios`, `linux`, `macos`, `windows`,
   ...
 - Environment (fourth word of the target triple): `gnu`, `msvc`, `musl`
-- WASM: `wasm32-bare` matches `wasm32-unknown-unknown`. `emscripten` also
-  matches that target as well as the emscripten targets.
+- WASM: `wasm32-bare` matches `wasm32-unknown-unknown`.
 - Pointer width: `32bit`, `64bit`
 - Endianness: `endian-big`
 - Stage: `stage1`, `stage2`