AArch64: Cleanup ifuncs

Cleanup ifuncs.  Remove uses of libc_hidden_builtin_def, use ENTRY rather than
ENTRY_ALIGN, remove unnecessary defines and conditional compilation.  Rename
strlen_mte to strlen_generic.  Remove rtld-memset.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
(cherry picked from commit 9fd34098)