ANDROID: 16K: Don't set padding vm_flags on 32-bit archs

vma_pad_fixup_flags() and is_mergable_pad_vma() were inadvertently
affecting the vm_flags on 32-bit arch, making some VMAs not mergable.

This causes zygote to crash as the Art GC's heap compaction fails.

The compaction depends on mremap() which will fail when operating on
a range that spans multiple VMAs [1]. This can happen now due to the
incorrect is_mergable_pad_vma() check.

Make all the pgsize_migration APIs no-ops in 32-bit architectures,
since Android only performs ELF segment extension in 64-bit archs.

[1] https://github.com/torvalds/linux/blob/v6.9/mm/mremap.c#L841-L843



Bug: 353667356
Change-Id: Id9b0076ef173d75a4afc85577355d340fce03e65
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
(cherry picked from commit f3437db87063f624f189e1cd38347a971fdd3fa0)