LoongArch: Define instruction formats for AM{SWAP/ADD}.{B/H} and DBAR

LoongArch: Define instruction formats for AM{SWAP/ADD}.{B/H} and DBAR

The 8 and 16 bit read-modify-write atomic instructions amadd.{b/h} and
amswap.{b/h} were newly added in the latest LoongArch Reference Manual,
define the instruction format and check whether support via CPUCFG.

Furthermore, define the instruction format for DBAR which will be used
to support BPF load-acquire and store-release instructions.

This is preparation for later patches.

Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>