Since GDB 13.1(GDB commit ea3352172), GDB LoongArch changed to use
fcc0-7 instead of fcc register. This commit partially reverts commit
2f149c75 (`target/loongarch: Update gdb_set_fpu() and gdb_get_fpu()`)
to match the behavior of GDB.

Note that it is a breaking change for GDB 13.0 or earlier, but it is
also required for GDB 13.1 or later to work.

Signed-off-by: Jiajie Chen <c@jia.je>
Acked-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230808054315.3391465-1-c@jia.je>
Signed-off-by: Song Gao <gaosong@loongson.cn>