AArch64: Fix instability in AdvSIMD tan

AArch64: Fix instability in AdvSIMD tan

Previously presence of special-cases in one lane could affect the
results in other lanes due to unconditional scalar fallback. The old
WANT_SIMD_EXCEPT option (which has never been enabled in libmvec) has
been removed from AOR, making it easier to spot and fix this. 4%
improvement in throughput with GCC 14 on Neoverse V1. This bug is
present as far back as 2.39 (where tan was first introduced).

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>