hadrian: optionally allow building text with simdutf support

This patch adds +text_simdutf hadrian flavour transformer to allow build text with simdutf support, which has been disabled for a long time (#20724 (closed)). This feature is still disabled by default, but building with +text_simdutf should allow downstream packagers and users that build GHC from source to opt-in this feature to improve UTF-8 decoding performance at run-time by harnessing the SIMD speedup.

Additionally, this is enabled for the wasm CI jobs, which is already tested to work, and is something we want to ship to end users.