Fix broken PERL_MEM_LOG under threads

Fix broken PERL_MEM_LOG under threads

This fixes GH #18341

There are problems with getenv() on threaded perls wchich can lead to
incorrect results when compiled with PERL_MEM_LOG.

Commit 0b83dfe6dd9b0bda197566adec923f16b9a693cd fixed this for some
platforms, but as Tony Cook, pointed out there may be
standards-compliant platforms that that didn't fix.

The detailed comments outline the issues and (complicated) full solution.