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

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

In gdb/testsuite/gdb.base/charset.c, the last argument is greater than 127
when call fill_run() in EBCDIC-US and IBM1047, but the type of string[] is
char, this will change the value due to sign extension.

For example, ebcdic_us_string[7] will be -63 instead of the original 193 in
EBCDIC-US.

Make the type of string[] as unsigned char to fix the following six failed
testcases:

  $ grep FAIL gdb/testsuite/gdb.sum
  FAIL: gdb.base/charset.exp: check value of parsed character literal in EBCDIC-US
  FAIL: gdb.base/charset.exp: check value of parsed string literal in EBCDIC-US
  FAIL: gdb.base/charset.exp: check value of escape that doesn't exist in EBCDIC-US
  FAIL: gdb.base/charset.exp: check value of parsed character literal in IBM1047
  FAIL: gdb.base/charset.exp: check value of parsed string literal in IBM1047
  FAIL: gdb.base/charset.exp: check value of escape that doesn't exist in IBM1047

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