ART: Workaround for ARM valgrind stack issues

The reports from pthreads and the builtins don't match up (compiled
code and thus actual stack pointer values haven't been tested, yet).
As a temporary workaround, implicitly disable the checks by storing
0x0 as the stack base.

Bug: 19127663
Bug: 29282211
Change-Id: Ib4170e222ca7e2ef92bf125b0a7409770ded7822