gdb: LoongArch: Implement the push_dummy_call gdbarch method

According to "Procedure Calling Convention" in "LoongArch ELF ABI
specification" [1], implement the push_dummy_call gdbarch method
as clear as possible.

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



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