[Perl/perl5] 8f2087: Make aTHXa an expression under no-threads

[Dagfinn Ilmari Mannsåker via perl5-changes]

  Branch: refs/heads/smoke-me/ilmari/better-noop
  Home:   https://github.com/Perl/perl5
  Commit: 8f2087639bf66b7ea6b177e1da6733d708278b41
      
https://github.com/Perl/perl5/commit/8f2087639bf66b7ea6b177e1da6733d708278b41
  Author: Dagfinn Ilmari Mannsåker <ilm...@ilmari.org>
  Date:   2021-09-21 (Tue, 21 Sep 2021)

  Changed paths:
    M perl.h

  Log Message:
  -----------
  Make aTHXa an expression under no-threads