Use free instead of xfree to free altstack

The altstack memory of a thread may be free'ed even after the VM is
destructed. After that, GC is no longer available, so calling xfree
may lead to a segfault.

This changeset uses the bare free function to free the altstack memory
instead of xfree. [Bug #18126]

signal.c

@@ -557,10 +557,13 @@ static int rb_sigaltstack_size_value = 0;
 void *
 rb_allocate_sigaltstack(void)
 {
+    void *altstack;
     if (!rb_sigaltstack_size_value) {
 	rb_sigaltstack_size_value = rb_sigaltstack_size();
     }
-    return xmalloc(rb_sigaltstack_size_value);
+    altstack = malloc(rb_sigaltstack_size_value);
+    if (!altstack) rb_memerror();
+    return altstack;
 }
 
 /* alternate stack for SIGSEGV */

vm_core.h

@@ -136,7 +136,7 @@
 void *rb_allocate_sigaltstack(void);
 void *rb_register_sigaltstack(void *);
 #  define RB_ALTSTACK_INIT(var, altstack) var = rb_register_sigaltstack(altstack)
-#  define RB_ALTSTACK_FREE(var) xfree(var)
+#  define RB_ALTSTACK_FREE(var) free(var)
 #  define RB_ALTSTACK(var)  var
 #else /* noop */
 #  define RB_ALTSTACK_INIT(var, altstack)