Move PAGE_MASK into <sys/user.h>.

I'm removing the TODO on the assumption that being compatible with glibc
is more useful than BSD. The new internal "bionic_page.h" header factors
out some duplication between libc and the linker.

Bug: http://b/22735893
Change-Id: I4aec4dcba5886fb6f6b9290a8f85660643261321

libc/bionic/libc_init_static.cpp

@@ -49,16 +49,10 @@
 #include "libc_init_common.h"
 #include "pthread_internal.h"
 
+#include "private/bionic_page.h"
 #include "private/bionic_tls.h"
 #include "private/KernelArgumentBlock.h"
 
-// Returns the address of the page containing address 'x'.
-#define PAGE_START(x)  ((x) & PAGE_MASK)
-
-// Returns the address of the next page after address 'x', unless 'x' is
-// itself at the start of a page.
-#define PAGE_END(x)    PAGE_START((x) + (PAGE_SIZE-1))
-
 extern "C" int __cxa_atexit(void (*)(void *), void *, void *);
 
 static void call_array(void(**list)()) {