target/mips: Add loongson-ext lsdc2 group of instructions

target/mips: Add loongson-ext lsdc2 group of instructions

LDC2/SDC2 opcodes have been rewritten as "load & store with offset"
group of instructions by loongson-ext ASE.

This patch add implementation of these instructions:

  gslbx: load 1 bytes to GPR
  gslhx: load 2 bytes to GPR
  gslwx: load 4 bytes to GPR
  gsldx: load 8 bytes to GPR
  gslwxc1: load 4 bytes to FPR
  gsldxc1: load 8 bytes to FPR
  gssbx: store 1 bytes from GPR
  gsshx: store 2 bytes from GPR
  gsswx: store 4 bytes from GPR
  gssdx: store 8 bytes from GPR
  gsswxc1: store 4 bytes from FPR
  gssdxc1: store 8 bytes from FPR

Details of Loongson-EXT is here:
https://github.com/FlyGoat/loongson-insn/blob/master/loongson-ext.md

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1602831120-3377-5-git-send-email-chenhc@lemote.com>