RFR: 8243470: [macos] bring back O2 opt level for unsafe.cpp

[Vladimir Kempik]

Hello
Please review a fix for JDK-8243470

Long time ago as part of JEP284: New HotSpot Build System this fix was applied to jdk9 https://bugs.openjdk.java.net/browse/JDK-8152666
At that time it was decided to lower optimisation level for unsafe.cpp from O2 to O1 only for clang on Macosx.

I suppose it was done due to issues in Set/Get<Object> helper functions where too optimistic optimisations were eliminating some null pointer checks. it was probably a clang bug.
That issue could be checked with test jdk/test/sun/misc/CopyMemory.java.

I believe that workaround (going from O2 to O1) produced this issue - https://bugs.openjdk.java.net/browse/JDK-8234963 (Thread.getStackTrace is slow with clang).
JDK-8234963 can only be seen on mac with libjvm compiled by clang.

Here I propose the patch which eliminates that workaround for clang 8+.

I have tested clang versions 8/9/9.1/10, all of them showed good results:
1) CopyMemory test passes fine on 11/14/15.
2) jdk11/jdk14 passed tck. Regression testing were good as well. jdk15: no new failures in tck.
3) The testRandom "benchmark" from 8234963 shows great improvements on my machine, going down from ~200 seconds to ~150 seconds (the newer clang the better result). For comparision, gcc built libjvm for jdk11 shows ~130 seconds on my machine.

The webrev: http://cr.openjdk.java.net/~vkempik/8243470/webrev.00/

getStackTrace benchmark: http://cr.openjdk.java.net/~vkempik/8243470/TestRandom.java

The bug: https://bugs.openjdk.java.net/browse/JDK-8243470

Thanks, Vladimir