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/stat.h

@@ -0,0 +1,54 @@
+/****************************************************************************
+ ****************************************************************************
+ ***
+ ***   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_STAT_H
+#define _LINUX_STAT_H
+
+#if !defined(__GLIBC__) || __GLIBC__ < 2
+
+#define S_IFMT 00170000
+#define S_IFSOCK 0140000
+#define S_IFLNK 0120000
+#define S_IFREG 0100000
+#define S_IFBLK 0060000
+#define S_IFDIR 0040000
+#define S_IFCHR 0020000
+#define S_IFIFO 0010000
+#define S_ISUID 0004000
+#define S_ISGID 0002000
+#define S_ISVTX 0001000
+
+#define S_ISLNK(m) (((m) & S_IFMT) == S_IFLNK)
+#define S_ISREG(m) (((m) & S_IFMT) == S_IFREG)
+#define S_ISDIR(m) (((m) & S_IFMT) == S_IFDIR)
+#define S_ISCHR(m) (((m) & S_IFMT) == S_IFCHR)
+#define S_ISBLK(m) (((m) & S_IFMT) == S_IFBLK)
+#define S_ISFIFO(m) (((m) & S_IFMT) == S_IFIFO)
+#define S_ISSOCK(m) (((m) & S_IFMT) == S_IFSOCK)
+
+#define S_IRWXU 00700
+#define S_IRUSR 00400
+#define S_IWUSR 00200
+#define S_IXUSR 00100
+
+#define S_IRWXG 00070
+#define S_IRGRP 00040
+#define S_IWGRP 00020
+#define S_IXGRP 00010
+
+#define S_IRWXO 00007
+#define S_IROTH 00004
+#define S_IWOTH 00002
+#define S_IXOTH 00001
+
+#endif
+
+#endif