target/loongarch: Update gdb_set_fpu() and gdb_get_fpu()

GDB LoongArch fpu use fcc register, update gdb_set_fpu()
and gdb_get_fpu() to match it.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220805033523.1416837-6-gaosong@loongson.cn>