Make prctl test check maps.

It turns out that sometimes this kernel bug can be present but not
trigger a crash. Modify the test to check for overlapping maps instead.

Bug: 32640542

Test: Verified that this fails on a device without the kernel patch
Test: (both bionic-unit-tests32 and CtsBionicTestCases32). Verified
Test: this passes on an angler.

Change-Id: Ia3fae3ee3d01f33d8414c1796fc3e942d5b56ec8