[AArch64][gas] Add support for Neoverse E1

[AArch64][gas] Add support for Neoverse E1

This adds support for the Neoverse E1 CPU [1] to gas.

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

[1] https://community.arm.com/processors/b/blog/posts/arm-neoverse-e1-platform-empowering-the-infrastructure-to-meet-next-generation-throughput-demands

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

    * config/tc-aarch64.c (aarch64_cpus): Add neoverse-e1.
    * doc/c-aarch64.texi (-mcpu): Document neoverse-e1 value.