[AArch64][gas] Add support for Neoverse N1

[AArch64][gas] Add support for Neoverse N1

This adds support for the Neoverse N1 [1] CPU to gas.
This was previously enabled under the Ares codename, which remains as
a valid option for -mcpu for compatibility reasons.

make check-gas passes on aarch64-none-elf.

[1] https://community.arm.com/processors/b/blog/posts/arm-neoverse-n1-platform-accelerating-the-transformation-to-a-scalable-cloud-to-edge-infrastructure

2019-02-21  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * config/tc-aarch64.c (aarch64_cpus): Add neoverse-n1.
    * doc/c-aarch64.texi (-mcpu): Document neoverse-n1 value.