Race when building ct.sym build tools

There seem to be a race when compiling build.tools.symbolgenerator.CreateSymbols:

[2020-11-03T07:21:12,118Z] Error: LinkageError occurred while loading main class build.tools.symbolgenerator.CreateSymbols
[2020-11-03T07:21:12,119Z] java.lang.ClassFormatError: Truncated class file
[2020-11-03T07:21:12,125Z] Gendata.gmk:69: recipe for target '/.../build/macosx-x64-open/support/javadoc-symbols/symbols' failed
[2020-11-03T07:21:12,125Z] make[3]: *** [/.../build/macosx-x64-open/support/javadoc-symbols/symbols] Error 1
[2020-11-03T07:21:12,127Z] make/Main.gmk:148: recipe for target 'jdk.javadoc-gendata' failed
[2020-11-03T07:21:12,127Z] make[2]: *** [jdk.javadoc-gendata] Error 2
[2020-11-03T07:21:12,127Z] make[2]: *** Waiting for unfinished jobs....

This is similar to JDK-8255620, and is possibly also caused by JDK-8206311.