gdb: testsuite: make string[] type as char in gdb.base/charset.c

This reverts the commit ff656e2e

 ("gdb: testsuite: fix failed
testcases in gdb.base/charset.exp").

The original test code has no problem. On an architecture where
char is signed, then both 'A' and ebcdic_us_string[7] will yield
-63, which makes the equality true. On an architecture where char
is unsigned, then both 'A' and ebcdic_us_string[7] will yield 193,
which also makes the equality true.

The test cases only failed on LoongArch. The default type of char
is signed char on LoongArch, like x86-64. But when use gdb print
command on LoongArch, the default type of char is unsigned char,
this is wrong, I will look into it later, sorry for that.

On LoongArch:

  $ cat test_char.c
  #include <stdio.h>

  int main()
  {
          char c1 = 193;
          unsigned char c2 = 193;

          printf("%d\n", c1);
          printf("%d\n", c1 == c2);

          return 0;
  }
  $ gcc test_char.c -o test_char
  $ ./test_char
  -63
  0

  (gdb) set target-charset EBCDIC-US
  (gdb) print 'A'
  $1 = 193 'A'
  (gdb) print /c 'A'
  $2 = 193 'A'
  (gdb) print /u 'A'
  $3 = 193
  (gdb) print /d 'A'
  $4 = -63
  (gdb) print /x 'A'
  $5 = 0xc1

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