Error out early on nonstandard JDK directory layouts

On Darwin, javac may be located in a nonstandard directory layout such as:
/System/Library/Frameworks/JavaVM.framework/Versions/Current/Commands/javac
The sed command to replace bin/javac with lib/tools.jar would fail to
match, resulting in the HOST_JDK_TOOLS_JAR being set to the javac path.
Since javac exists, the checks for missing tools.jar would all pass, and
javac would be added to the classpath instead of tools.jar, and causing
hard to debug errors about missing com.sun.javadoc when building doclava.

Change the sed command to replace /javac$, which should always be found,
with /../lib/tools.jar.

Change-Id: I5072f04636a5c14b3aeaa3a5cc3b366feae89c37