Add a --javac_extdir flag to enable configuring the javac -extdirs flag

The -extdirs flag sets a search path to the directory containing the JDK's
extension classes, and should always be set explicitly when cross-compiling. If
it is unset, the ext directory of the host JDK will be used instead.

javac requires that -extdirs be a list of directories. The blaze --javac_extdir
flag takes the label of a filegroup with the 'path' attribute set, and passes
the directory path through to JavaBuilder.

--
MOS_MIGRATED_REVID=94001278

tools/jdk/BUILD

diff --git a/tools/jdk/BUILD b/tools/jdk/BUILD
index ee6ffd7..a3d4fe1 100644
--- a/tools/jdk/BUILD
+++ b/tools/jdk/BUILD
@@ -34,6 +34,11 @@
 )
 
 filegroup(
+    name = "extdir",
+    srcs = ["//external:extdir"],
+)
+
+filegroup(
     name = "langtools",
     srcs = ["//external:langtools"],
 )