malloc: Turn tst-mallocstate into a non-internal test

compat_symbol_reference no longer needs tests-internal.  Do not build
the test at all for newer targets, so that no spurious UNSUPPORTED
result is generated.  Use compat_symbol_reference for
__malloc_initialize_hook as well, eliminating the need for -rdynamic.

Reviewed-by: DJ Delorie <dj@redhat.com>