MFC r340984: 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.

Merged to stable/12 as libc's ifunc use has been merged there; ifunc use
has not been merged to releng/12.0 so this change does not need to be
either.