tools/runqslower: Fix LDFLAGS and add LDLIBS support

Actually use previously defined LDFLAGS during build and add support for
LDLIBS to link extra standalone libraries e.g. 'argp' which is not provided
by musl libc.

Fixes: 585bf464 ("tools: runqslower: Add EXTRA_CFLAGS and EXTRA_LDFLAGS support")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/bpf/20240723003045.2273499-1-tony.ambardar@gmail.com