Be more careful to avoid including system headers after perl.h

Be more careful to avoid including system headers after perl.h

Commit 121d2d3d70 included simd.h into pg_wchar.h. This caused a problem
on Windows, since Perl has "#define free" (referring to globals), which
breaks the Windows' header. To fix, move the static inline function
definitions from plperl_helpers.h, into plperl.h, where we already
document the necessary inclusion order. Since those functions were the
only reason for the existence of plperl_helpers.h, remove it.

First reported by Justin Pryzby

Diagnosis and review by Andres Freund, patch by myself per suggestion
from Tom Lane

Discussion: https://www.postgresql.org/message-id/20220826115546.GE2342%40telsasoft.com