When building in the environment described below, we see this error:

-----
21:18:48 ld: 0711-407 ERROR: Symbol .sigaction
21:18:48 Visibility is not allowed on a reference to an imported symbol.
21:18:48 Lib-java.base.gmk:139: recipe for target '/home/u0020236/workspace/PullRequest-Sanity-JDK11-aix_ppc-64_cmprssptrs-OpenJDK11/build/aix-ppc64-normal-server-release/support/modules_libs/java.base/libjsig.so' failed
-----

After analysis, the belief is that this is caused by JNIEXPORT and JNIIMPORT not being defined. Without this, almost no symbols are exported from shared libraries due to use of -qvisibility=hidden as specified in make/lib/LibCommon.gmk.

Note: One solution for this problem (from which the above quotation was borrowed) was laid out in the OpenJDK Extensions for OpenJ9 repository, here: https://github.com/ibmruntimes/openj9-openjdk-jdk/pull/27