Use declarations from cxxabi.h where available

Change-Id: Ie92c9654c7ccc5cd4acde728b35311f251f740ac
Reviewed-on: https://gerrit.libreoffice.org/64963
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

bridges/source/cpp_uno/gcc3_linux_arm/share.hxx

@@ -96,11 +96,15 @@ namespace CPPU_CURRENT_NAMESPACE
 
     // -- following decl from libstdc++-v3/libsupc++/unwind-cxx.h and unwind.h
 
+#if !HAVE_CXXABI_H_CXA_ALLOCATE_EXCEPTION
     extern "C" void *__cxa_allocate_exception(
         std::size_t thrown_size ) throw();
+#endif
+#if !HAVE_CXXABI_H_CXA_THROW
     extern "C" void __cxa_throw (
         void *thrown_exception, std::type_info *tinfo,
         void (*dest) (void *) ) __attribute__((noreturn));
+#endif
 
 }