core.current.stubs: Support compiling under OpenJDK 9 javac -target 1.9

This CL adds
 --patch-module=java.base=.
to the javac command line used to compile the stub sources; since those
sources are compiled with an empty bootclasspath (--system=none),
javac 9 otherwise doesn't count the compiled sources as part of
java.base, and therefore doesn't find the package java.lang within it.
The associated error message ("Unable to find package java.lang in
classpath or bootclasspath") is slightly vague/misleading, but the
--patch-module argument is similarly used in the core-all build target.

Test: USE_R8=true EXPERIMENTAL_USE_OPENJDK=true make checkbuild
Bug: 38177569

Change-Id: I54992284f4f95c91e82e6fa83b84f27a2b3a1435