Properly run tst-spawn5 directly [BZ #28067]

Change tst-spawn5.c to handle tst-spawn5 without optional path to ld.so,
--library-path nor the library path when glibc is configured with
--enable-hardcoded-path-in-tests.  This fixes BZ #28067.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>