ANDROID: GKI: Add pageflags for OEM

Add PG_oem_reserved_x pageflags for OEM on 64-bit platform.

These new flags can be used to indicate different states of page.
For example, we can use these pageflags to identify anonymous pages
at different ZRAM compression rates.
These flags will be used in conjunction with different Android
application states. Therefore, I think this is not a generic
modification for all Linux devices and is not suitable for upstream
submission.

Bug: 336964184
Change-Id: Ie68087248866af63ac516d96e3af85222e7a0b50
Signed-off-by: Xiaofeng Yuan <yuanxiaofeng@vivo.com>