RFR: JDK-8269650: Optimize gc-locker in [Get|Release]StringCritical for latin string

[Hamlin Li]

Currently, JNI GetStringCritical locks gc locker for all strings including latin and non-latin until ReleaseStringCritical.
But for latin, it's not necessary to still lock gc locker after GetStringCritical, as it's copied anyway whether obj pining is supported or not, so it's fine to unlock gc locker after GetStringCritical.

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Commit messages:
 - optimize gc-locker in GetStringCritical for latin str

Changes: https://git.openjdk.java.net/jdk/pull/4637/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4637&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269650
  Stats: 4 lines in 1 file changed: 3 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4637.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4637/head:pull/4637

PR: https://git.openjdk.java.net/jdk/pull/4637