[testsuite] Add missing require-effective-target alloca

Add missing require-effect-target alloca directives.

Tested on nvptx.

gcc/testsuite/ChangeLog:

	* gcc.dg/Warray-bounds-63.c: Add require-effective-target alloca.
	* gcc.dg/Warray-bounds-66.c: Same.
	* gcc.dg/atomic/stdatomic-vm.c: Same.

gcc/testsuite/gcc.dg/Warray-bounds-63.c

@@ -1,7 +1,8 @@
 /* PR middle-end/94195 - missing warning reading a smaller object via
    an lvalue of a larger type
    { dg-do compile }
-   { dg-options "-O2 -Wall" } */
+   { dg-options "-O2 -Wall" }
+   { dg-require-effective-target alloca } */
 
 typedef __INT16_TYPE__ int16_t;
 typedef __SIZE_TYPE__  size_t;

gcc/testsuite/gcc.dg/Warray-bounds-66.c

@@ -1,6 +1,7 @@
 /* PR middle-end/82608 - missing -Warray-bounds on an out-of-bounds VLA index
  { dg-do compile }
- { dg-options "-O2 -Wall -Wno-uninitialized -ftrack-macro-expansion=0" } */
+ { dg-options "-O2 -Wall -Wno-uninitialized -ftrack-macro-expansion=0" }
+ { dg-require-effective-target alloca } */
 
 #include "range.h"
 

gcc/testsuite/gcc.dg/atomic/stdatomic-vm.c

@@ -2,6 +2,7 @@
    with side effects.  */
 /* { dg-do run } */
 /* { dg-options "-std=c11 -pedantic-errors" } */
+/* { dg-require-effective-target alloca } */
 
 #include <stdatomic.h>