Do not allow to load ET_DYN object with DF_1_PIE flag set.

Linkers are supposed to mark PIE binaries with DF_1_PIE, such binary
cannot be correctly and usefully loaded neither by dlopen(3) nor as a
dependency of other object. For instance, we cannot do anything
useful with COPY relocations, among other things.

Glibc already added similar restriction.

Requested and reviewed by: emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D25086