mesa/main: drop use_legacy_math_rules

mesa/main: drop use_legacy_math_rules

Seems the only thing that really needs this is fpow(0, 0), which should
return NaN, but then gets multiplied with zero. Let's fix that by doing
a bcsel instead of fmul to select the result here. While we're at it,
get rid of the fabs for stop, which isn't needed.

This fixes a piglits failure for most (if not all?) drivers that doesn't
support legacy math rules.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22789>