ANDROID: 16K: Use MADV_DONTNEED to save VMA padding pages.

When performing LOAD segment extension, the dynamic linker knows what
portion of the VMA is padding. In order for the kernel to implement
mitigations that ensure app compatibility, the extent of the padding
must be made available to the kernel.

To achieve this, reuse MADV_DONTNEED on single VMAs to hint the padding
range to the kernel. This information is then stored in vm_flag bits.
This allows userspace (dynamic linker) to set the padding pages on the
VMA without a need for new out-of-tree UAPI.

Bug: 330117029
Bug: 327600007
Bug: 330767927
Bug: 328266487
Bug: 329803029
Change-Id: I3421de32ab38ad3cb0fbce73ecbd8f7314287cde
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>