[8u] RFR: 8252975: [8u] JDK-8252395 breaks the build for --with-native-debug-symbols=internal

[Severin Gehwolf]

Hi,

Please review this 8u (jdk8u/jdk8u-dev tree) fix for JDK-8252395 that
I've pushed today. Thanks for Zhengyu Gu for noticing it. The pushed
fix added the java.debuginfo and unpack.debuginfo make targets on the
condition of ENABLE_DEBUG_SYMBOLS=true, which is insufficient. It needs
another check on POST_STRIP_CMD which is set to non-empty for --with-
native-debug-symbols={external,zipped}, and is indeed empty for --with-
native-debug-symbols=internal. For the --with-native-debug-symbols=none 
case we have ENABLE_DEBUG_SYMBOLS=false.

Bug: https://bugs.openjdk.java.net/browse/JDK-8252975
webrev: https://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8252975/01/webrev/

Testing: Builds with --with-native-debug-symbols={none,internal,external,zipped} on Linux x86_64

OK?

Thanks,
Severin