optimize CLEAR_ERRSV

optimize CLEAR_ERRSV

-gv_add_by_type is special only for non-GVs, and add AV to GV, otherwise
it is just assigning to GP slot (in this case through GvSV), which
CLEAR_ERRSV already did in another branch, so inline the gv_add_by_type
-dont compute GvSV multiple times, GvSV contains 2 derefs, after this patch
it will contains just 1 deref ("*svp") without an offset (deref without
offset is smaller in x86 machine code than deref with offset)
-SvREFCNT_dec_NN for efficiency
-move SvPOK_only closer to SvMAGICAL, this ensures SvFLAGS is read and
written only once, not 2 reads and 1 write, this is specifically for
RISC-ish cpus
-the goto clresv_newemptypv is because VC optimizer in -O1 didn't combine
the branches

perl521.dll VC 2003 before .text section size in machine code bytes
0xc8a63, after 0xc88e3