RFR(XS) 8222482: [Graal] Update java-allocation-instrumenter.jar handling in graalunit README.md

[Ekaterina Pavlova]

Hi All,

Please review the change which updates test/hotspot/jtreg/compiler/graalunit/README.md.
One more auxiliary script test/hotspot/jtreg/compiler/graalunit/downloadLibs.sh has been also added.

    JBS: https://bugs.openjdk.java.net/browse/JDK-8222482
webrev: http://cr.openjdk.java.net/~epavlova/8222482/webrev.00/index.html

thanks,
-katya