Declare __page_shift and __page_size with C linkage.

__page_shift and __page_size were accidentally declared in unistd.h with
C linkage - their implementation needs to use the same linkage.

Going forward, though, let's stop the inlining madness and let's kill
the non-standard __getpageshift(). This patch takes getpagesize(3) out
of line and removes __getpageshift but fixes __page_shift and __page_size
for backwards binary compatibility.

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