rtld: Add support for arm64 variant pcs

The aarch64 ELF spec has support for a variant of the normal procedure
call standard that doesn't follow the normal register convention, e.g.
using more registers as arguments, or different register state is
preserved.

Add support to rtld to handle this. As we don't know which registers
need to be preserved disable lazy binding for these functions.

Reviewed by: kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44869