fix Warning: used $at without ".set noat"

When code is assembled that uses register $1 the following warning
is produced:

  mips-linux-gnu-as -O0 -mips32 -o t0032-lw_sw.elf t0032-lw_sw.asm
  t0032-lw_sw.asm: Assembler messages:
  t0032-lw_sw.asm:11: Warning: used $at without ".set noat"

Register $1 corresponds to $at.  Eliminate use of $1 to quiet this
message.  Update test .check files.