Build interim-langtools for HotSpot only if Graal is enabled

Many HotSpot developers make only the "hotspot" target, and copy the resulting libjvm.so into an already-build JDK image.

HotSpot requires interim-langtools only when Graal is enabled. When Graal is disabled, we can avoid building interim-langtools. This improves the build time of "make hotspot" by about 20 seconds, or about 15%:

Old:
$ make clean
$ time make hotspot
....
real 1m57.905s
user 42m22.524s
sys 3m7.372s

New:
$ make clean
$ time make hotspot
....
real 1m39.916s
user 41m59.984s
sys 3m3.188s