Pass OpenJDK 8's bootclasspath for host tools targeting <= 1.8.

Host (as opposed to hostdex) tools compile and run against OpenJDK's
core libraries. Before this CL, the core libraries of the default
toolchain were always used, even when targeting an earlier language
version.

This meant that code that uses APIs from a later version of OpenJDK
than corresponded to LOCAL_JAVA_LANGUAGE_VERSION would compile, but
would fail to run under that earlier version of OpenJDK. It also
meant that calls to existing APIs might be reinterpreted; for
example, the return type of java.nio.ByteBuffer.clear() changed from
Buffer in OpenJDK 8 to ByteBuffer in OpenJDK 9. At compile time, this
was noted via the warning:
  bootstrap class path not set in conjunction with -source 1.8

After this CL, when targeting a language version <= 1.8 (which is
always the case when building with OpenJDK 8), some of OpenJDK 8's
core library/tools jars are now passed on the bootclasspath. The
decision to include the bootclasspath argument when building with
OpenJDK 8 was somewhat arbitrary, but has the advantage that we
discover any issues before we switch to OpenJDK 9.

Even when compiling with OpenJDK 9, use of OpenJDK 9 APIs will now
fail at compile time rather than at runtime; calls to existing APIs
will now be interpreted in OpenJDK 8 rather than 9 fashion. For
example, this means that dx and host-side CTS tests built with
OpenJDK 9 javac -target 1.8 will be runnable under OpenJDK 8.

Bug: 70521453
Bug: 70862583
Test: Checked that the bootclasspath argument was passed
      in the javac invocation targeting 1.8 during:
      make showcommands compatibility-common-util-hostsidelib
Test: make checkbuild

Change-Id: I9b6081edfdd2c3e9a450ae8a39c4e32c3d2cda92