fs/binfmt_elf.c: smaller code generation around auxv vector fill

Filling auxv vector as array with index (auxv[i++] = ...) generates
terrible code.  "saved_auxv" should be reworked because it is the worst
member of mm_struct by size/usefullness ratio but do it later.

Meanwhile help gcc a little with *auxv++ idiom.

Space savings on x86_64:

	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-127 (-127)
	Function                                     old     new   delta
	load_elf_binary                             5470    5343    -127

Link: http://lkml.kernel.org/r/20191208172301.GD19716@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>