SUMMARY
mips: Add support Loongson special instruction 

Implement Loonson MIPS specific instructions :

 Multiplication, the result is written to the general purpose register:
	GSMULT, GSDMULT, GSMULTU, GSDMULTU
 Division, quotient write general purpose register:
	GSDIV, GSDDIV, GSDIVU, GSDDIVU
 Division, remainder writes to the general purpose register:
	GSMOD, GSDMOD, GSMODU, GSDMODU
 Logical right shift:
	PSLLW, PSLLH, PSRLH, PSRLW
 Arithmetic right shift:
	PSRAW
 After register fs is negated, register ft is bitwise and:
	PANDN
 Unpack:
	PUNPCKHHW, PUNPCKLBH, PUNPCKHBH
 Low 32 bits combined into 64 bits:
	PUNPCKLWD
 High 32 bits combined into 64 bits:
	PUNPCKHWD
 Take word with offset:
	GSLDX, GSLBX, GSLHX, GSLWX
 Memory bytes with offset:
	GSSDX, GSSBX, GSSHX, GSSWX	
 Dual source register stores words:
	GSSQ, GSSQC1
 Dual source register take words:
	GSLQ

This patch must be committed after 
https://bugs.kde.org/show_bug.cgi?id=429282

STEPS TO REPRODUCE
1. make and make install
2. gcc testcase.c -o testcase -g -march=loongson3a
3. run ./valgrind testcase

OBSERVED RESULT


EXPECTED RESULT


SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION

$ cat /proc/cpuinfo 
system type		: generic-loongson-machine
machine			: loongson,generic
processor		: 0
cpu model		: ICT Loongson-3 V0.4  FPU V0.1
model name		: Loongson-3A R4 (Loongson-3A4000)
CPU MHz			: 1800.00
BogoMIPS		: 3594.02
wait instruction	: yes
microsecond timers	: yes
tlb_entries		: 2112
extra interrupt vector	: yes
hardware watchpoint	: yes, count: 0, address/irw mask: []
isa			: mips1 mips2 mips3 mips4 mips5 mips32r1 mips32r2 mips64r1 mips64r2

For more instruction information view ：
http://www.loongson.cn/uploadfile/cpu/3A3000/Loongson3A3000_3B3000user2.pdf