Simplify embedvar.h, removing a level of macro indirection for PL_* variables.

Simplify embedvar.h, removing a level of macro indirection for PL_* variables.

For the default (non-multiplicity) configuration, PERLVAR*() macros now
directly expand their arguments to tokens such as C<PL_defgv>, instead of
expanding to C<PL_Idefgv>. This removes over 350 lines from F<embedvar.h>,
which defined macros to map from C<PL_Idefgv> to C<PL_defgv> and so forth.