target/loongarch: Add fixed point load/store instruction translation

This includes:
- LD.{B[U]/H[U]/W[U]/D}, ST.{B/H/W/D}
- LDX.{B[U]/H[U]/W[U]/D}, STX.{B/H/W/D}
- LDPTR.{W/D}, STPTR.{W/D}
- PRELD
- LD{GT/LE}.{B/H/W/D}, ST{GT/LE}.{B/H/W/D}
- DBAR, IBAR

Signed-off-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220606124333.2060567-8-yangxiaojuan@loongson.cn>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>