Unify NDK platform headers.

This commit removes duplication of system headers per platform, i.e. remove
copies of common headers like <stdlib.h> from arch-arm/usr/include and arch-x86/usr/include
and move them to a common include directory. More specifically:

- common headers:        android-N/arch-A/usr/include --> android-N/include
- arch-specific headers: android-N/arch-A/usr-include --> android-N/arch-A/include
- arch-specific libs:    android-N/arch-A/usr/lib     --> android-N/arch-A/lib

Change-Id: Ifdba5038d108901931f3e3a7c14ebe6270d2d276

NOTE: This also contains careful separation of API-level-specific headers.
      For example, platforms/android-5/include/pthread.h contains new function
      declarations that are not available when using platforms/android-3/include/pthread.h

NOTE: The NDK's build-platforms.sh script has been updated to understand the new
      layout. This change in develeopment/ndk does not change the layout of
      platform files under $NDK/platforms after build-platforms.sh is called.

ndk/platforms/android-3/include/linux/stringify.h

@@ -0,0 +1,18 @@
+/****************************************************************************
+ ****************************************************************************
+ ***
+ ***   This header was automatically generated from a Linux kernel header
+ ***   of the same name, to make information necessary for userspace to
+ ***   call into the kernel available to libc.  It contains only constants,
+ ***   structures, and macros generated from the original header, and thus,
+ ***   contains no copyrightable information.
+ ***
+ ****************************************************************************
+ ****************************************************************************/
+#ifndef __LINUX_STRINGIFY_H
+#define __LINUX_STRINGIFY_H
+
+#define __stringify_1(x) #x
+#define __stringify(x) __stringify_1(x)
+
+#endif