[11u] do not disable c99 on Solaris

https://bugs.openjdk.java.net/browse/JDK-8215710

removed the disabling of C99 on Solaris in jdk13.
Reason was :
"Currently we disable C99 in the Solaris build by setting -xc99=%none%.
This differs from a lot of other build environments like gcc/Linux or VS2013/2017 on Windows where C99 features work.
We should remove this difference on Solaris and remove or replace the setting. "
It would be helpful to have this downported to jdk11, because it would make downporting changes easier (brings build settings of the releases closer together).