NativeLibraryHelper: Remove use of hardcoded PAGE_SIZE 4096

bionic hard codes the PAGE_SIZE macro as 4096. This is going away as
Android begins to support larger page sizes. Remove the usage of this
assumption from NativeLibraryHelper source; use instead getpagesize()
which provides the real pagesize.

Test: mma
Bug: 295082607
Change-Id: If59fdc17379adda1dcd6e78e754e7e17ef49423d
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>