re-enabled Checkstyle with the release of 6.0 that supports Java 8; fixed existing Checkstyle warnings

--HG--
rename : graal/com.oracle.graal.graph/.checkstyle_checks.xml.disabled => graal/com.oracle.graal.graph/.checkstyle_checks.xml
rename : graal/com.oracle.truffle.api/.checkstyle_checks.xml.disabled => graal/com.oracle.truffle.api/.checkstyle_checks.xml
Truffle: added PrimitiveValueProfile.
Truffle: CallTarget to RootCallTaget in OptimizationFailedException.
Truffle: do not throw permament compiler exceptions into guest languages.