libc: Use pthread stack info for RLIMIT_STACK

Currently, RLIMIT_STACK is 64 KB.  This is too small for the JVM which
limits stack size to RLIMIT_STACK if a VMA range that covers address
found in '__libc_stack_end' in /proc/self/maps.

In preparation for procfs support, switch to pthread_attr_getstacksize()
for RLIMIT_STACK.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

libc/libc.cc

@@ -42,9 +42,15 @@ int getrlimit(int resource, struct rlimit *rlim)
 {
     auto set = [=] (rlim_t r) { rlim->rlim_cur = rlim->rlim_max = r; };
     switch (resource) {
-    case RLIMIT_STACK:
-        set(64*1024); // FIXME: something realer
+    case RLIMIT_STACK: {
+        pthread_attr_t attr;
+        pthread_attr_init(&attr);
+        size_t stacksize;
+        pthread_attr_getstacksize(&attr, &stacksize);
+        set(stacksize);
+        pthread_attr_destroy(&attr);
         break;
+    }
     case RLIMIT_NOFILE:
         set(1024*10); // FIXME: larger?
         break;