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

Follow the Make change in I9b6081edfdd2c3e9a450ae8a39c4e32c3d2cda92
to explicitly pass the OpenJDK 8 bootclasspath when targeting <= 1.8.

Bug: 70862583
Test: java_test.go
Test: javap -c -p out/soong/.intermediates/external/guava/guava/linux_glibc_common/javac/classes/com/google/common/hash/AbstractStreamingHashFunction\$AbstractStreamingHasher.class  | grep ByteBuffer.flip
      shows java/nio/Buffer return type in signature.

Change-Id: Ief66bbf6e3a4220b3afb2e02009bd0157d4c7fae