hadrian: add +text_simdutf flavour transformer to allow building text with simdutf

This patch adds a +text_simdutf flavour transformer to hadrian to
allow downstream packagers and users that build from source to opt-in
simdutf support for text, in order to benefit from SIMD speedup at
run-time. It's still disabled by default for the time being.

(cherry picked from commit 44e2abfb)