Fix cross-building on Linux/aarch64

Add necessary bits to detect ELF format on Linux/aarch64; note that
Linux calls it aarch64 where we would typically call it arm64 (uname -m)

Reviewed by: arichardson, emaste, imp
Sponsored by: Ampere Computing LLC
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D32542