Avoid deadlock with PERL_MEM_LOG

[khwilliamson]

 Avoid deadlock with PERL_MEM_LOG
 
 This fixes GH #18341
 
 The Perl wrapper for getenv() was changed in 5.32 to allocate memory to
 squirrel safely away the result of the wrapped getenv() call.  It does
 this while in a critical section so as to make sure another thread can't
 interrupt it and destroy it.
 
 Unfortunately, when Perl is compiled for debugging memory problems and
 has PERL_MEM_LOG enabled, that allocation causes a recursive call to
 getenv() for the purpose of checking an environment variable to see how
 to log that allocation.  And hence it deadlocks trying to enter the
 critical section.
 
 There are various solutions.  One is to use or emulate a general semaphore
 instead of a binary one.  This is effectively what
 PL_lc_numeric_mutex_depth does for another mutex, and the code for that
 could be used as a template.
 
 But given that this is an extreme edge case which requires Perl to be
 specially compiled to enable this feature which is used only for
 debugging, a much simpler, if less safe if it were to ever be used in
 production, solution should suffice.  Tony Cook suggested just avoiding
 the wrapper for this particular purpose.

[tonycoz]

Why not just call getenv() here? That's what happens anyway, and doesn't cost every other Perl_mortal_getenv() the (minor) cost of a string comparison.

[khwilliamson]

That is a much better solution than mine. Updated to do what you say.