Add basic Darwin support to libcorkscrew.

This might not seem like much, but it's already better than Mac OS'
backtrace(3).

(cherry picked from commit 2a46f6293f19b9d6100187fbd53088088e671b88)

Change-Id: I197b06c4c19f728ad6f194b2f473bd9dcd99b8a6
diff --git a/libcorkscrew/test.cpp b/libcorkscrew/test.cpp
new file mode 100644
index 0000000..6f7be3b
--- /dev/null
+++ b/libcorkscrew/test.cpp
@@ -0,0 +1,71 @@
+#include <corkscrew/backtrace.h>
+#include <corkscrew/symbol_table.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int do_backtrace(float /* just to test demangling */) {
+  const size_t MAX_DEPTH = 32;
+  backtrace_frame_t* frames = (backtrace_frame_t*) malloc(sizeof(backtrace_frame_t) * MAX_DEPTH);
+  ssize_t frame_count = unwind_backtrace(frames, 0, MAX_DEPTH);
+  fprintf(stderr, "frame_count=%d\n", (int) frame_count);
+  if (frame_count <= 0) {
+    return frame_count;
+  }
+
+  backtrace_symbol_t* backtrace_symbols = (backtrace_symbol_t*) malloc(sizeof(backtrace_symbol_t) * frame_count);
+  get_backtrace_symbols(frames, frame_count, backtrace_symbols);
+
+  for (size_t i = 0; i < (size_t) frame_count; ++i) {
+    char line[MAX_BACKTRACE_LINE_LENGTH];
+    format_backtrace_line(i, &frames[i], &backtrace_symbols[i],
+                          line, MAX_BACKTRACE_LINE_LENGTH);
+    if (backtrace_symbols[i].symbol_name != NULL) {
+      // get_backtrace_symbols found the symbol's name with dladdr(3).
+      fprintf(stderr, "  %s\n", line);
+    } else {
+      // We don't have a symbol. Maybe this is a static symbol, and
+      // we can look it up?
+      symbol_table_t* symbols = NULL;
+      if (backtrace_symbols[i].map_name != NULL) {
+        symbols = load_symbol_table(backtrace_symbols[i].map_name);
+      }
+      const symbol_t* symbol = NULL;
+      if (symbols != NULL) {
+        symbol = find_symbol(symbols, frames[i].absolute_pc);
+      }
+      if (symbol != NULL) {
+        int offset = frames[i].absolute_pc - symbol->start;
+        fprintf(stderr, "  %s (%s%+d)\n", line, symbol->name, offset);
+      } else {
+        fprintf(stderr, "  %s (\?\?\?)\n", line);
+      }
+      free_symbol_table(symbols);
+    }
+  }
+
+  free_backtrace_symbols(backtrace_symbols, frame_count);
+  free(backtrace_symbols);
+  free(frames);
+  return frame_count;
+}
+
+struct C {
+  int g(int i);
+};
+
+__attribute__ ((noinline)) int C::g(int i) {
+  if (i == 0) {
+    return do_backtrace(0.1);
+  }
+  return g(i - 1);
+}
+
+extern "C" __attribute__ ((noinline)) int f() {
+  C c;
+  return c.g(5);
+}
+
+int main() {
+  f();
+  return 0;
+}