This patch adds support to gas for -mcpu=cortex-a76 in the usual way.
make check-gas passes without problems.

2018-06-08  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    * config/tc-arm.c (arm_cpus): Add Cortex-A76 entry.
    * doc/c-arm.texi (-mcpu): Document cortex-a76.