refactor gv.c:Perl_newGP

refactor gv.c:Perl_newGP

This commit fixes a scenario was strlen("") was called unnecessarily.
Replaced with 0. Also various func calls were rearranged for more calls
to happen near the beginning to maximize use of volatile registers
towards the end for PERL_HASH. PERL_HASH was moved to be closer to the
first usage of var hash. Setting gp_line to 0 was removed since the block
was just calloced and is already 0. Filling of gp.gp_egv was moved early
so var gv on C stack might get reused by compiler optimizer to store
something else to decrease the stack frame size of Perl_newGP.
PERL_ARGS_ASSERT_NEWGP was moved to be outside of an ifdef.

Also see commits 128165928a7 , 19bad6733a8 , 1df5f7c1950 , f4890806d3 .