tools/libs/gnttab: Fix PAGE_SIZE redefinition error

tools/libs/gnttab: Fix PAGE_SIZE redefinition error

If PAGE_SIZE is already defined in the system (e.g. in /usr/include/limits.h
header) then gcc will trigger a redefinition error because of -Werror. This
patch replaces usage of PAGE_* macros with XC_PAGE_* macros in order to avoid
confusion between control domain page granularity (PAGE_* definitions) and
guest domain page granularity.

The exception is in osdep_xenforeignmemory_map() where we need the system page
size to check whether the PFN array should be allocated with mmap() or with
dynamic allocation.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Acked-by: Ian Jackson <iwj@xenproject.org>