Enable inlining of java_lang_Class::is_primitive

Moving Class::is_primitive to javaClasses.inline.hpp is a small micro-optimization on a microbenchmark that exercise java.lang.reflect.Array.newInstance with C1 or C2 with intrinsics disabled.

The method is also so trivial - sans the assert - that inlining the call at the 50-60 callsites in Hotspot actually shrinks the size of the VM by a tiny amount.