[PPC64] Performance regression after JDK-8290025

JDK-8290025 has enabled usage of nmethod entry barriers regardless of the selected GC. This causes a noticeable performance drop (about 3% in SPEC jvm98, jvm2008, and a bit more in jbb2015). PPC64 suffers more than x86 because the nmethod entry barrier contains a memory barrier for GCs which patch nmethods during a concurrent phase.

However, not all GCs perform patching of nmethods while the Java Threads are running, so, the memory barrier is not always needed.