Declare __page_shift and __page_size with C linkage

__page_shift and __page_size are defined in asm/page.h with
C linkage - their implementation needs to use the same linkage.

The different linkage types cause a compiler error with clang.
Fixing them seems to be a good idea regardless of the compiler.

Change-Id: I35ed66a08989ced1db422eb03e4d154a5d6b5bda
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>