RFR: 8299343: Windows: Invalid thread_native_entry declaration

[Kim Barrett]

Please review this change to the Windows version of thread_native_entry.  It
is changed from being inconsistently declared (both file-scoped and external)
to consistently being a private static member function in os::win32.  It's
signature is also changed to match where it's used, eliminating the need for a
function pointer cast.

Testing:
mach5 tier1

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

Commit messages:
 - use static member function

Changes: https://git.openjdk.org/jdk/pull/11787/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299343
  Stats: 9 lines in 2 files changed: 6 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11787.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11787/head:pull/11787

PR: https://git.openjdk.org/jdk/pull/11787