Build changes that allow the modules on arm64.

• Move the required kernel compiler flags from Makefile.arm64 to kern.mk.
• Build arm64 modules as PIC; non-PIC relocations in .o for shared object output cannot be handled.
• Do not try to install aarch64 symlink.
• A hack for arm64 to avoid ld -r stage. See the comment for the explanation. Some functionality is lost, like ctf handling, but hopefully will be restored after newer linker is available.

Reviewed by: andrew, emaste
Tested by: andrew (on real hardware)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3796