AArch64: Improve strlen_asimd performance (bug 25824)

AArch64: Improve strlen_asimd performance (bug 25824)

Optimize strlen using a mix of scalar and SIMD code.  On modern micro
architectures large strings are 2.6 times faster than existing
strlen_asimd and 35% faster than the new MTE version of strlen.

On a random strlen benchmark using small sizes the speedup is 7% vs
strlen_asimd and 40% vs the MTE strlen.  This fixes the main strlen
regressions on Cortex-A53 and other cores with a simple Neon unit.

Rename __strlen_generic to __strlen_mte, and select strlen_asimd when
MTE is not enabled (this is waiting on support for a HWCAP_MTE bit).

This fixes big-endian bug 25824. Passes GLIBC regression tests.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>