Patch libbase to allow it to be compiled with Emscripten

In addition to these patches an empty sys/cdefs.h must be provided in the
include path for compilation to be successful.

GetExecutablePath will trigger an abort since there isn't really a resonable
value we could return in that environment.

Test: make libbase and build Emscripten
Change-Id: I1a39e9cbd168ea8c83d3705464eb0283106fddca
diff --git a/file.cpp b/file.cpp
index 69ee69f..e433a07 100644
--- a/file.cpp
+++ b/file.cpp
@@ -488,6 +488,8 @@
   if (result == 0 || result == sizeof(path) - 1) return "";
   path[PATH_MAX - 1] = 0;
   return path;
+#elif defined(__EMSCRIPTEN__)
+  abort();
 #else
 #error unknown OS
 #endif