7012642: G1: JumbleGC002 test aborts with segmentation violation due to uncaught stack overflow

With recent G1 allocation path changes, the value of StackShadowPages in fast debug builds of the JVM, is no longer large enough to prevent the JVM C++ code from touching the stack guard pages. Increase the value of StackShadowPages to a suitable value.

Reviewed-by: ysr, tonyp, coleenp