RFR: 8247396: PCH should be disabled for opt-size feature with clang after JDK-8246751

[jiefu(傅杰)]

Hi all,

JBS:    https://bugs.openjdk.java.net/browse/JDK-8247396
Webrev: http://cr.openjdk.java.net/~jiefu/8247396/webrev.00/

-O3 is used for clang on MacOS after JDK-8246751.
But it breaks the builds of VMs with opt-size feature when PCH is enabled.

The reason is that the PCH was built with -Os while some other files were built with -O3, which causes an error with clang.
It would be better to disable the PCH for opt-size builds.

Thanks a lot.
Best regards,
Jie