ANDROID: 16K: Introduce /sys/kernel/mm/pgsize_miration/enabled

Migrating from 4kB to 16kB page-size in Android requires first making
the platform page-agnostic, which involves increasing Android-ELFs'
max-page-size (p_align) from 4kB to 16kB.

Increasing the ELF max-page-size was found to cause compatibility issues
in apps that use obfuscation or depend on the ELF segments being mapped
based on 4kB-alignment.

Working around these compatibility issues involves both kernel and
userspace (dynamic linker) changes.

Introduce a knob for userspace (dynamic linker) to determine whether the
kernel supports the mitigations needed for page-size migration compatibility.

The knob also allows for userspace to turn on or off these mitigations
by writing 1 or 0 to /sys/kernel/mm/pgsize_miration/enabled:

    echo 1 > /sys/kernel/mm//pgsize_miration/enabled  # Enable
    echo 0 > /sys/kernel/mm//pgsize_miration/enabled  # Disable

Bug: 330117029
Bug: 327600007
Bug: 330767927
Bug: 328266487
Bug: 329803029
Change-Id: I9ac1d15d397b8226b27827ecffa30502da91e10e
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>