[macOS] Use PAGE_SIZE macro instead of getpagesize() to build on macOS big sur.

[doneria-anjali]

During psutil library installation on mac OS 11.0 (aka Big Sur), during building, the setup fails with error:
psutil/_psutil_osx.c:516:20: error: implicit declaration of function 'getpagesize' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
int pagesize = getpagesize();

Enhancement is to support installation of psutil on macOS 11.0 using python-pip by using PAGE_SIZE instead of getpagesize(). PAGE_SIZE is a macro that gives you the vm_page_size global.

[giampaolo]

Please make a PR.