ART: Relax heap verification failure count

Changes the atomic fail_count variable used in heap verification to be
a plain variable. It is used thread private state and can be updated
and read with the default C++ memory ordering.

Bug: 31023171
Test: art/test.py --host -j32
Change-Id: I16ad3e138797b9900ecf719ab22c0b2b03e53c33
1 file changed