gdbserver: LoongArch: Simplify code with register number macros

Move "enum loongarch_regnum" to gdb/arch/loongarch.h so that the
macro definitions can be used in gdbserver/linux-loongarch-low.cc
to simplify the code.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>