remove PL_patchlevel from S_minus_v

|SvPVX(vstringify(PL_patchlevel))| is the same string as
|"v" PERL_VERSION_STRING|. No need to go through the overhead of using a
version object. Instead of creating a SV, then further manipulating it.
Create and manipulate it at the same time with Perl_newSVpvf_nocontext or
newSVpvn. "num_len >= level_len " will be folded away by the compiler,
previously, since both lengths came from SVs, they were not const
technically. A very smart compiler might see strncmp/strnEQ takes all
const arguments, and will optimize that away also. VC 2003 didnt do that.
Change SvREFCNT_dec to SvREFCNT_dec_NN for obvious reasons.

There should not be any user visible changes to -v with this patch.
switches.t contains a regexp for -v already so no further tests were added.

This patch is part of #116296.