Don't use run-via-rtld-prefix for anything other than tests

Don't use run-via-rtld-prefix for anything other than tests

run-via-rtld-prefix checks whether the program to be run is a static
test and skips if it is.  This is fine, except that it assumes that
the program to be run is the second $^, which is true only for tests.

This change creates an rtld-prefix, which is simply the dynamic linker
prefix with the necessary arguments and uses that in the non-test
targets.