aarch64: Add AmpereComputing emag to tunable cpu list

Emag is a 64-bit CPU core released by AmpereComputing.

Add its name to cpu list, and corresponding macro as utilities for
later IFUNC dispatch.

    * manual/tunables.texi (Tunable glibc.cpu.name): Add emag.
    * sysdeps/unix/sysv/linux/aarch64/cpu-features.c (cpu_list):
    Add emag.
    * sysdeps/unix/sysv/linux/aarch64/cpu-features.h (IS_EMAG):
    New macro.