RFR 8218705: Test sun/tools/jcmd/TestJcmdDefaults.java fails on Linux

[Daniil Titov]

Please review the change that fix the test failure. 

The problem here is that the code in sun.tools.common.ProcessArgumentMatcher.check() method was supposed to fallback to the default mechanism for retrieving the main class name if  sun.tools.common.ProcessHelper (recently introduced in JDK-8205654) failed to detect it, but it does not. The change fixes that problem.

Webrev: http://cr.openjdk.java.net/~dtitov/8218705/webrev.01/
Bug: https://bugs.openjdk.java.net/browse/JDK-8218705

Thanks!
--Daniil