aarch64: MTE compatible strlen

Introduce an Arm MTE compatible strlen implementation.

The existing implementation assumes that any access to the pages in
which the string resides is safe.  This assumption is not true when
MTE is enabled.  This patch updates the algorithm to ensure that
accesses remain within the bounds of an MTE tag (16-byte chunks) and
improves overall performance on modern cores. On cores with less
efficient Advanced SIMD implementation such as Cortex-A53 it can
be slower.

Benchmarked on Cortex-A72, Cortex-A53, Neoverse N1.

Co-authored-by: Wilco Dijkstra <wilco.dijkstra@arm.com>