[foreign-jextract] RFR: 8247547: jextract generated Cstring should use foreign.CSupport string utility methods

[Athijegannathan Sundararajan]

Changed Cstring to use CSupport string utils.

-------------

Commit messages:
 - 8247547: jextract generated Cstring should use foreign.CSupport string utility methods

Changes: https://git.openjdk.java.net/panama-foreign/pull/204/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/204/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247547
  Stats: 40 lines in 1 file changed: 1 ins; 33 del; 6 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/204.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/204/head:pull/204

PR: https://git.openjdk.java.net/panama-foreign/pull/204