SafeStack: Add builtins to read unsafe stack top/bottom

[vlad.tsyrklevich]

Summary

Introduce built-ins to read the unsafe stack top and bottom. The unsafe
stack top is required to implement garbage collection scanning for
Oilpan. Currently there is already a built-in 'get_unsafe_stack_start'
to read the bottom of the unsafe stack, but I chose to duplicate this
API because 'start' is ambiguous (e.g. Oilpan uses WTF::GetStackStart to
read the safe stack top.)

[pcc]

LGTM

lib/safestack/safestack.cc

 // initialization early.
 extern "C" {
 __attribute__((section(".preinit_array"),
                used)) void (*__safestack_preinit)(void) = __safestack_init;
 }
 #endif
 
 extern "C"
-    __attribute__((visibility("default"))) void *__get_unsafe_stack_start() {
+    __attribute__((visibility("default"))) void *__get_unsafe_stack_bottom() {
   return unsafe_stack_start;
 }
 
 extern "C"
+    __attribute__((visibility("default"))) void *__get_unsafe_stack_top() {
+  return (char*)unsafe_stack_start + unsafe_stack_size;
+}
+
+extern "C"
+    __attribute__((visibility("default"), alias("__get_unsafe_stack_bottom")))
+    void *__get_unsafe_stack_start();
+
+extern "C"
     __attribute__((visibility("default"))) void *__get_unsafe_stack_ptr() {
   return __safestack_unsafe_stack_ptr;
 }