elf_common.h: Add AArch64 Memtag ABI constants

Obtained from the Memtag ABI Extension to ELF for the Arm® 64-bit
Architecture (AArch64) [1]

[1] https://github.com/ARM-software/abi-aa/blob/main/memtagabielf64/memtagabielf64.rst#dynamic-section

Reviewed by: andrew
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55486