testsuite: Add necessary dejagnu directives to pr115815_0.c

I have received an email from the Linaro infrastructure that the test
gcc.dg/lto/pr115815_0.c which I added is failing on arm-eabi and I
realized that not only it is missing dg-require-effective-target
global_constructor but actually any dejagnu directives at all, which
means it is unnecessarily running both at -O0 and -O2 and there is an
unnecesary run test too.  All fixed by this patch.

I have not actually verified that the failure goes away on arm-eabi
but have very high hopes it will.  I have verified that the test still
checks for the bug and also that it passes by running:

  make -k check-gcc RUNTESTFLAGS="lto.exp=*pr115815*"

gcc/testsuite/ChangeLog:

2024-10-14  Martin Jambor  <mjambor@suse.cz>

	* gcc.dg/lto/pr115815_0.c: Add dejagu directives.

gcc/testsuite/gcc.dg/lto/pr115815_0.c

@@ -1,3 +1,7 @@
+/* { dg-lto-options {{-O2 -flto}} }  */
+/* { dg-lto-do link } */
+/* { dg-require-effective-target global_constructor } */
+
 int a;
 volatile int v;
 volatile int w;