[foreign-jextract] RFR: CSupport::getSystemLinker should not be called from internal API points

[Maurizio Cimadamore]

Some jextract internal classes call the external getSystemLinker method, which then does a check on the
`foreign.restricted` property (which is disabled when jextract runs). Trusted internal API points should use the
internal factory for the system linker instead.

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

Commit messages:
 - Fix issue with CSupport.getSystemCLinker() being called from internal API points.

Changes: https://git.openjdk.java.net/panama-foreign/pull/198/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/198/webrev.00
  Stats: 18 lines in 3 files changed: 17 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/198.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/198/head:pull/198

PR: https://git.openjdk.java.net/panama-foreign/pull/198