aarch64: Use future HWCAP2_MTE in ifunc resolver

Make glibc MTE-safe on systems where MTE is available. This allows
using heap tagging with an LD_PRELOADed malloc implementation that
enables MTE. We don't document this as guaranteed contract yet, so
glibc may not be MTE safe when HWCAP2_MTE is set (older glibcs
certainly aren't). This is mainly for testing and debugging.

The HWCAP flag is not exposed in public headers until Linux adds it
to its uapi. The HWCAP value reservation will be in Linux 5.9.