It seems a regression from recent change to GensrcVarHandles.gmk, VARHANDLES_GENSRC_DIR is not defined:


```
Exception in thread "main" java.nio.file.AccessDeniedException: /VarHandleGuards.java
at java.base/sun.nio.fs.UnixException.translateToIOException(UnixException.java:90)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:106)
at java.base/sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:111)
at java.base/sun.nio.fs.UnixFileSystemProvider.newFileChannel(UnixFileSystemProvider.java:213)
at java.base/sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:244)
at java.base/java.nio.file.spi.FileSystemProvider.newOutputStream(FileSystemProvider.java:426)
at java.base/java.nio.file.Files.newOutputStream(Files.java:215)
at java.base/java.nio.file.Files.newBufferedWriter(Files.java:2722)
at build.tools.methodhandle.VarHandleGuardMethodGenerator.main(VarHandleGuardMethodGenerator.java:173)
gmake[3]: *** [gensrc/GensrcVarHandles.gmk:165: /VarHandleGuards.java] Error 1
gmake[3]: *** Waiting for unfinished jobs....
gmake[2]: *** [make/Main.gmk:99: java.base-gensrc-src] Error 2
gmake[2]: *** Waiting for unfinished jobs....
```