libgcc: fix SEH C++ rethrow semantics [PR113337]

SEH _Unwind_Resume_or_Rethrow invokes abort directly if
_Unwind_RaiseException doesn't manage to find a handler for the rethrown
exception; this is incorrect, as in this case std::terminate should be
invoked, allowing an application-provided terminate handler to handle
the situation instead of straight crashing the application through
abort.

The bug can be demonstrated with this simple test case:
===
static void custom_terminate_handler() {
    fprintf(stderr, "custom_terminate_handler invoked\n");
    std::exit(1);
}

int main(int argc, char *argv[]) {
    std::set_terminate(&custom_terminate_handler);
    if (argc < 2) return 1;
    const char *mode = argv[1];
    fprintf(stderr, "%s\n", mode);
    if (strcmp(mode, "throw") == 0) {
        throw std::exception();
    } else if (strcmp(mode, "rethrow") == 0) {
        try {
            throw std::exception();
        } catch (...) {
            throw;
        }
    } else {
        return 1;
    }
    return 0;
}
===

On all gcc builds with non-SEH exceptions, this will print
"custom_terminate_handler invoked" both if launched as ./a.out throw or
as ./a.out rethrow, on SEH builds instead if will work as expected only
with ./a.exe throw, but will crash with the "built-in" abort message
with ./a.exe rethrow.

This patch fixes the problem, forwarding back the error code to the
caller (__cxa_rethrow), that calls std::terminate if
_Unwind_Resume_or_Rethrow returns.

The change makes the code path coherent with SEH _Unwind_RaiseException,
and with the generic _Unwind_Resume_or_Rethrow from libgcc/unwind.inc
(used for SjLj and Dw2 exception backend).

libgcc/ChangeLog:

	PR libgcc/113337

	* unwind-seh.c (_Unwind_Resume_or_Rethrow): forward
	_Unwind_RaiseException return code back to caller instead of
	calling abort, allowing __cxa_rethrow to invoke std::terminate
	in case of uncaught rethrown exception

libgcc/unwind-seh.c

@@ -395,9 +395,9 @@ _Unwind_Reason_Code
 _Unwind_Resume_or_Rethrow (struct _Unwind_Exception *exc)
 {
   if (exc->private_[0] == 0)
-    _Unwind_RaiseException (exc);
-  else
-    _Unwind_ForcedUnwind_Phase2 (exc);
+    return _Unwind_RaiseException (exc);
+
+  _Unwind_ForcedUnwind_Phase2 (exc);
   abort ();
 }