Add cxa_guard functions for C++ ABI compatibility

src/SConscript

@@ -10,7 +10,8 @@ import subprocess
 files = [
     'boot.s',
     'Main.cc',
-    'Console.cc'
+    'Console.cc',
+    'cxa.cc',
 ]
 
 toolchain_bin = Dir(os.environ['POLKA_TOOLCHAIN']).Dir('bin')

src/cxa.cc

@@ -0,0 +1,49 @@
+/* cxa.cc
+ * vim: set tw=80:
+ * Eryn Wells <eryn@erynwells.me>
+ */
+/**
+ * Implements some functions required by the C++ ABI so that local static
+ * variables can be initialized properly.
+ */
+
+namespace __cxxabiv1 {
+
+// 64-bit integer type to service as mutex-like guard.
+__extension__ typedef int __guard __attribute__((mode(__DI__)));
+
+extern "C" {
+
+/*
+ * TODO: The functions below are required by the C++ ABI and should be thread
+ * safe. Since Polka currently has no threads, it should be fine to do this
+ * without any actual locking. Once/If I add threading, I need to come back here
+ * and add an actual mutex with a test-and-set.
+ *
+ * See http://wiki.osdev.org/C%2B%2B#Local_Static_Variables_.28GCC_Only.29
+ * See the Intel XCHG instruction
+ */
+
+int
+__cxa_guard_acquire(__guard *g)
+{
+    return !*(g);
+}
+
+
+void
+__cxa_guard_release(__guard *g)
+{
+    *g = 1;
+}
+
+
+void
+__cxa_guard_abort(__guard *)
+{
+    // TODO: What would be useful to do here?
+}
+
+}
+
+} /* namespace __cxxabiv1 */