Add Linux LoongArch64 execute command payload

[bcoles]

Verification

$ ./msfvenom -p linux/loongarch64/exec CMD="echo hello world>/tmp/asdf" -f elf -o exec.elf && chmod +x exec.elf 
[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, selecting arch: loongarch64 from the payload
No encoder specified, outputting raw payload
Payload size: 108 bytes
Final size of elf file: 228 bytes
Saved as: exec.elf

$ /home/user/qemu/build/qemu-loongarch64 -strace ./exec.elf 
3127512 execve("/bin/sh",{"/bin/sh","-c","echo hello world>/tmp/asdf",NULL})