gdb: LoongArch: Add LBT extension support

Loongson Binary Translation (LBT) is used to accelerate binary
translation, which contains 4 scratch registers (scr0 to scr3),
x86/ARM eflags (eflags) and x87 fpu stack pointer (ftop). This
patch support gdb to fetch/store these registers.

Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn> #  Framework
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>   #  Detail Optimizes
Signed-off-by: Hui Li <lihui@loongson.cn>             #  Error Fixes
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>