main: Don't add int to string

main: Don't add int to string

clang shows this warning:

  /home/emaisin/src/binutils-gdb/gdb/main.c:227:56: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
                char *tmp_sys_gdbinit = xstrdup (SYSTEM_GDBINIT + datadir_len);
                                                 ~~~~~~~~~~~~~~~^~~~~~~~~~~~~
  /home/emaisin/src/binutils-gdb/gdb/main.c:227:56: note: use array indexing to silence this warning
                char *tmp_sys_gdbinit = xstrdup (SYSTEM_GDBINIT + datadir_len);
                                                                ^
                                                 &              [            ]

It's quite easy to get rid of it by using &foo[len] instead of foo + len.
I think this warning is relevant to keep enabled, because it can be an
easy mistake to do.

This warning is already discussed here in GCC bugzilla:

  https://gcc.gnu.org/ml/gcc-patches/2017-06/msg00729.html

and a patch series for it was submitted very recently.

gdb/ChangeLog:

	* main.c (get_init_files): Replace "SYSTEM_GDBINIT +
	datadir_len" with "&SYSTEM_GDBINIT[datadir_len]".