RFR: 8196988 (Resolve disabled warnings for implicit-fallthrough gcc option)

[Srinivas Dama]

Hi,

Please review http://cr.openjdk.java.net/~sdama/8196988/webrev.00/ 
for https://bugs.openjdk.java.net/browse/JDK-8196988

Note: gcc 7.3 compiler emits warnings for implicit fall-through switch cases,but
these warnings are disabled earlier.I have enabled these warnings and fixed in sources.

Regards,
Srinivas