target/mips: Add loongson-ext lswc2 group of instructions (Part 2)

target/mips: Add loongson-ext lswc2 group of instructions (Part 2)

LWC2 & SWC2 have been rewritten by Loongson EXT vendor ASE
as "load/store quad word" and "shifted load/store" groups of
instructions.

This patch add implementation of these instructions:

  gslwlc1: similar to lwl but RT is FPR instead of GPR
  gslwrc1: similar to lwr but RT is FPR instead of GPR
  gsldlc1: similar to ldl but RT is FPR instead of GPR
  gsldrc1: similar to ldr but RT is FPR instead of GPR
  gsswlc1: similar to swl but RT is FPR instead of GPR
  gsswrc1: similar to swr but RT is FPR instead of GPR
  gssdlc1: similar to sdl but RT is FPR instead of GPR
  gssdrc1: similar to sdr but RT is FPR instead of GPR

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>
Message-Id: <1602831120-3377-4-git-send-email-chenhc@lemote.com>
[PMD: Reuse t1 on MIPS32, reintroduce t2/fp0]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>