gdb: LoongArch: Add support for static data member in struct

gdb: LoongArch: Add support for static data member in struct

As described in C++ reference [1], static data members are not part
of objects of a given class type. Modified compute_struct_member ()
to ignore static data member so that we can get the expected result.

loongson@linux:~$ cat test.c
#include<stdio.h>
struct struct_01 { static unsigned a; float b;};
unsigned struct_01::a = 66;
struct struct_01 struct_01_val = { 99.00 };
int check_arg_struct(struct struct_01 arg)
  {
    printf("arg.a = %d\n", arg.a);
    printf("arg.b = %f\n", arg.b);
    return 0;
  }
int main()
  {
    check_arg_struct(struct_01_val);
    return 0;
  }
loongson@linux:~$ g++ -g test.c -o test++
loongson@linux:~$ gdb test++

Without this patch:
...
(gdb) start
...
(gdb) p check_arg_struct(struct_01_val)
arg.a = 66
arg.b = 0.000000
$1 = 0

With this patch:
...
(gdb) start
...
(gdb) p check_arg_struct(struct_01_val)
arg.a = 66
arg.b = 99.000000
$1 = 0

[1] https://learn.microsoft.com/en-us/cpp/cpp/static-members-cpp?view=msvc-170

Signed-off-by: Hui Li <lihui@loongson.cn>
Reviewed-By: Tom Tromey <tom@tromey.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>