[libc] Revert #73704 and subsequent fixes #73984, #74026 (#74355)

The test cases of mincore require getting correct page size from OS. As
`sysconf` is not functioning correctly, these patches are implemented in
a somewhat confusing way. We revert such patches and will reintroduce
mincore after we correct sysconf.

This reverts 54878b80, 985c0d19 and 418a3a45.