gdb: LoongArch: Implement the get_syscall_number gdbarch method

In the current code, the feature 'catch syscall' is not supported
on LoongArch, implement the "get_syscall_number" gdbarch method to
get the system call number from the register a7.

Without this patch:

(gdb) catch syscall
The feature 'catch syscall' is not supported on this architecture yet.

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