Error out on attempt to link amd64 kernel with old binutils linker

As of r333461 we require ifunc support to link a working amd64 kernel.
The default in-tree bootstrap linker is lld and it has the required
support, as does any modern out-of-tree binutils linker. The in-tree
GNU ld is from binutils 2.17.50 and it does not have ifunc support,
so produce an error rather than a broken kernel.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15378