Thomas Backlund reported that the perf build was failing on the Mageia 7
distro, that is because it uses:

  cat /tmp/build/perf/feature/test-disassembler-four-args.make.output
  /usr/bin/ld: /usr/lib64/libbfd.a(plugin.o): in function `try_load_plugin':
  /home/iurt/rpmbuild/BUILD/binutils-2.32/objs/bfd/../../bfd/plugin.c:243:
  undefined reference to `dlopen'
  /usr/bin/ld:
  /home/iurt/rpmbuild/BUILD/binutils-2.32/objs/bfd/../../bfd/plugin.c:271:
  undefined reference to `dlsym'
  /usr/bin/ld:
  /home/iurt/rpmbuild/BUILD/binutils-2.32/objs/bfd/../../bfd/plugin.c:256:
  undefined reference to `dlclose'
  /usr/bin/ld:
  /home/iurt/rpmbuild/BUILD/binutils-2.32/objs/bfd/../../bfd/plugin.c:246:
  undefined reference to `dlerror'
  as we allow dynamic linking and loading

Mageia 7 uses these linker flags:
  $ rpm --eval %ldflags
    -Wl,--as-needed -Wl,--no-undefined -Wl,-z,relro -Wl,-O1 -Wl,--build-id -Wl,--enable-new-dtags

So add -ldl to this feature LDFLAGS.
Reported-by: Thomas Backlund <tmb@mageia.org>
Tested-by: Thomas Backlund <tmb@mageia.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Link: https://lkml.kernel.org/r/20190501173158.GC21436@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>