druntime-test: memoryerror_stackoverflow try harder to avoid optimizations (#21742)

gdc and ldc2 with llvm-20 optimize away the array declarations leading
to an infinite cycle.

See-Also: ldc-developers/ldc#4969

Signed-off-by: Andrei Horodniceanu <a.horodniceanu@proton.me>

Author: the-horo

druntime/test/exceptions/src/memoryerror_stackoverflow.d

@@ -1,16 +1,19 @@
 import etc.linux.memoryerror;
+import core.volatile;
 
 pragma(inline, false):
 
 void f(ref ubyte[1024] buf)
 {
     ubyte[1024] cpy = buf;
+    volatileStore(&cpy[0], 1);
     g(cpy);
 }
 
 void g(ref ubyte[1024] buf)
 {
     ubyte[1024] cpy = buf;
+    volatileStore(&cpy[0], 2);
     f(cpy);
 }