MIPS: memset: Limit excessive `noreorder' assembly mode use

Rewrite to use the `reorder' assembly mode and remove manually scheduled
delay slots except where GAS cannot schedule a delay-slot instruction
due to a data dependency or a section switch (as is the case with the EX
macro).  No change in machine code produced.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
[paul.burton@mips.com:
  Fix conflict with commit 932afdee ("MIPS: Add Kconfig variable for
  CPUs with unaligned load/store instructions")]
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20834/
Cc: Ralf Baechle <ralf@linux-mips.org>