AArch64: support .cpu directive

Add support for the AArch64 .cpu directive.  This is a slightly involved
directive since the parameter is actually a variable encoded string.  The
general structure is:

  <cpu>[[+-]<feature>]*

We now map some of the supported string names for features for internal
representation of feature flags.  If we encounter one which we do not support,
bail out as we cannot validate the assembly any longer.

Resolves PR27010.

llvm-svn: 265240