gdb: LoongArch: Add vector extensions support

Add LoongArch's vector extensions support, which including
128bit LSX (i.e., Loongson SIMD eXtension) and 256bit LASX
(i.e., Loongson Advanced SIMD eXtension). This patch support
gdb to fetch/store vector registers.

Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>