Move the temporary definitions for PERLVAR* macros inside #ifdef MULTIPLICITY

Move the temporary definitions for PERLVAR* macros inside #ifdef MULTIPLICITY

Under MULTIPLICITY, intrpvar.h is included "early", as:

struct interpreter {
#  include "intrpvar.h"
};

with local definitions of PERLVAR{,A,I,IS,ISC} that generate output text for the
"variables" as structure members. Additionally, under PERL_GLOBAL_STRUCT
"perlvars.h" is included within struct perl_vars { ... }.

Move the definition/undefining of these 5 macros to within the
#ifdef MULTIPLICITY block, to clarify the limited intent of their scope.

Move some additional related PERL_GLOBAL_STRUCT setup to within the block.