Skip IRELATIVE relocations when loader processes ELF files.

ifuncs can be only called in the (early boot) kernel environment, so
postpone resolving until early stage of the kernel boot. This commit
is performed in advance to make loaders on most machines updated
before ifuncs appear in the kernels.

Reviewed by: emaste, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D13838