target/mips: Add loongson-ext lswc2 group of instructions (Part 1)

target/mips: Add loongson-ext lswc2 group of instructions (Part 1)

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:

  gslq: load 16 bytes to GPR
  gssq: store 16 bytes from GPR
  gslqc1: load 16 bytes to FPR
  gssqc1: store 16 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>
Message-Id: <1602831120-3377-3-git-send-email-chenhc@lemote.com>
[PMD: Restrict t1 variable to TARGET_MIPS64, remove unused t2/fp0]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>