feat(detect_targets): enable HWY_WASM by default on WebAssembly (fixes #2831)

[jdymitarai]

Problem

As reported in #2831, hwy::DispatchedTarget previously returned HWY_EMU128 on WebAssembly unless the -msimd128 compiler flag was passed explicitly, because HWY_BASELINE_WASM checked defined(__wasm_simd128__).

Solution

Enable HWY_WASM by default on HWY_ARCH_WASM without requiring -msimd128, as discussed in #2831. WASM SIMD (simd128) is now widespread across modern web browsers and runtimes.

Developers building a non-SIMD fallback WebAssembly binary can cleanly opt out by specifying HWY_DISABLED_TARGETS=HWY_WASM (or HWY_COMPILE_ONLY_SCALAR), which falls back to HWY_EMU128 / HWY_SCALAR.

Fixes #2831.

[jan-wassenberg]

Unfortunately this is nontrivial to land, we have some internal projects which turn out to break when enabling SIMD by default, which is why #2851 was rolled back.

How much of an issue is it for you that -msimd128 is required to be passed?