Do not install GNU ld if lld is /usr/bin/ld

GNU binutils ld.bfd 2.17.50 does not support ifuncs and produces broken
binaries when ifuncs are in use. When LLD_IS_LD is default we have an
ifunc-capable system linker and can just avoid installing ld.bfd.

Reported by: theraven
Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18340