testsuite: Fix up pr113617 test for darwin [PR113617]

testsuite: Fix up pr113617 test for darwin [PR113617]

The test attempts to link a shared library, and apparently Darwin doesn't
allow by default for shared libraries to contain undefined symbols.

The following patch just adds dummy definitions for the symbols, so that
the library no longer has any undefined symbols at least in my linux
testing.
Furthermore, for target { !shared } targets (like darwin until the it is
fixed in target-supports.exp), because we then link a program rather than
shared library, the patch also adds a dummy main definition so that it
can link.

2024-03-08  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/113617
	PR target/114233
	* g++.dg/other/pr113617.C: Define -DSHARED when linking with -shared.
	* g++.dg/other/pr113617-aux.cc: Add definitions for used methods and
	templates not defined elsewhere.