provide PAGE_SIZE as a constant value of 8192 on or1k

according to Stefan Kristiansson, or1k page size is not actually
variable and the value of 8192 is part of the ABI.

arch/or1k/bits/limits.h

@@ -1,5 +1,6 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define PAGE_SIZE 8192
 #define LONG_BIT 32
 #endif