arm64: Provide ifunc HWCAP structure definitions

IFUNC structure is based on Section 9.4.1 "GNU C Library IFUNC interface"
from "System V ABI for the Arm 64-bit Architecture (AArch64)", 2025Q1.
(https://github.com/ARM-software/abi-aa/releases/download/2025Q1/sysvabi64.pdf)

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D54598

(cherry picked from commit 449339bdba2470eded4d55c41b084cfc8f5ef73a)