gdb: LoongArch: Handle variadic arguments

gdb: LoongArch: Handle variadic arguments

According to LoongArch ELF ABI specification [1], variadic arguments
are passed in GARs in the same manner as named arguments. And after
a variadic argument has been passed on the stack, all future arguments
will also be passed on the stack, i.e., the last argument register may
be left unused due to the aligned register pair rule. long double data
tpye is passed in an aligned GAR pair, the first register in the pair
is even-numbered.

[1] https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html

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