util: Add a _mesa_i64roundevenf() helper.

This always returns a int64_t, translating to _mesa_lroundevenf on
systems where long is 64-bit, and llrintf where "long long" is needed.

Fixes: 594fc0f8 ("mesa: Replace F_TO_I() with _mesa_lroundevenf().")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit b59914e1)