Increase the size of the shadow call stack guard region to 16MB.

Increasing the size of the guard region helps with the security of SCS,
but it's blocked on landing [1], which in turn is blocked on landing
[2]. Once those two CLs land we will be able to land this one.

[1] https://android-review.googlesource.com/c/platform/frameworks/av/+/837745
[2] https://android-review.googlesource.com/c/platform/bionic/+/818973

Bug: 118642754
Change-Id: I35409cbb6bfcd77e632567dd755376e345cfe67b