arm64: Enable SHA256 instruction in the kernel

[allanjude]

Summary

This enables the existing support for ARM64 SHA2 instructions in
userland (used by libmd) to be used in the kernel and various modules
like: crypto, ipsec, geli, zfs

Sponsored by: Ampere Computing LLC
Sponsored by: Klara Inc.

[andrew]

Is this safe if the caller doesn't have a floating point context? It could trash userspace floating point registers.

sys/conf/files.arm64

147

We also need -DARM64_SHA2 on sha256c.c. It might be easiest to add via CFLAGS.sha256c.c+=... in Makefile.arm64 and the module Makefile.

[allanjude]

Update to fix build error