TestMemProtChg_FLAKY fails assertion is_readable_without_exception_try(pc, 1)

This test fails consistently on the AArch64 nightly build machine. It hits the following assertion:

`<../build_debug-internal-64/suite/tests/bin/security-common.TestMemProtChg_FLAKY (7354). Internal Error: DynamoRIO debug check failure: ../src/core/vmareas.c:8201 is_readable_without_exception_try(pc, 1)`

http://dynamorio.org/CDash/testDetails.php?test=322307&build=26586
http://dynamorio.org/CDash/testDetails.php?test=321619&build=26542
http://dynamorio.org/CDash/testDetails.php?test=311514&build=26014

As @egrimley said in #2417, it seems like this test only fails on certain AArch64 implementations. On a system using a different AArch64 implementation to the nightly builder, the test consistently passes.