remove 1 read of interp var from PUSHMARK

PL_markstack_ptr was read once to do the ++ and comparison. Then after
the markstack_grow call, or not, depending on the branch. The code reads
PL_markstack_ptr a 2nd time. It has to be reread in case (or always does)
markstack_grow reallocs the mark stack. markstack_grow has a void retval.
That is a waste of a register. Let us put it to use to return the new
PL_markstack_ptr. In markstack_grow the contents that will be assigned to
PL_markstack_ptr are already in a register. So let the I32* flow out from
markstack_grow to its caller.

In VC2003 32 bit asm, mark_stack_entry is register eax. The retval of
markstack_grow is in eax. So the assignment "=" in
"mark_stack_entry = markstack_grow();" has no overhead. Since the other,
not extend branch, is function call free,
"(mark_stack_entry = ++PL_markstack_ptr)" assigns to eax. Ultimatly with
this patch a 3 byte mov instruction is saved for each instance of PUSHMARK,
and 1 interp var read is removed. I observed 42 callers of markstack_grow
with my disassembler, so theoretically 3*42 bytes of machine code was
removed for me.

Perl_pp_pushmark dropped from 0x2b to 0x28 bytes of x86 VC 2003
machine code. [perl #122034]