x86/asm/64: Clean up memset16(), memset32(), memset64() assembly constraints in <asm/string_64.h>

- Use "+" constraint modifier,
  simplify inputs and output lists,
  delete dummy variables with meaningless names,

  "&" only makes sense in complex assembly creating constraints on
  intermediate registers. But 1 instruction assemblies don't have
  inner body so to speak.

- Write "rep stos*" on one line:
  Rep prefix is integral part of x86 instruction.  I'm not sure why
  people separate "rep" with newline.

  Uros Bizjak adds context: "some archaic assemblers rejected 'rep
  insn' on one line. I have checked that the minimum required
  binutils-2.25 assembles this without problems."

- Use __auto_type for maximum copy pasta experience,

- Reformat a bit to make everything looks nicer.

Note that "memory" clobber is too much if "n" is known at compile time.
However,

	"=m" (*(T(*)[n])s)

doesn't work because -Wvla even if "n" is compile time constant:

	if (BCP(n)) {
		rep stos
		: "=m" (*(T(*)[n])s)
	} else {
		rep stosw
		: "memory"
	}

The above doesn't work.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20240314165715.31831-1-adobriyan@gmail.com