Work around VNET and DPCPU related panics on aarch64

lld >= 14 and recent GNU ld can relax adrp+add and adrp+ldr
instructions, which breaks VNET and DPCPU when used in modules.

Until VNET and DPCPU can be fixed to deal with these relaxed
instructions, disable linker relaxation for now.

PR: 264094
Reviewed by: markj
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41156