eal: replace page sizes enum with a set of constants

eal: replace page sizes enum with a set of constants

Clang on Windows follows MS ABI where enum values are limited to 2^31-1.
Enum rte_page_sizes has members valued above this limit, which get
wrapped to zero, resulting in compilation error (duplicate values in
enum). Using MS ABI is mandatory for Windows EAL to call Win32 APIs.

Remove rte_page_sizes and replace its values with #define's.
This enumeration is not used in public API, so there's no ABI breakage.
Announce API changes for 20.08 in documentation.

Suggested-by: Jerin Jacob <jerinjacobk@gmail.com>
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>