Werror=format-security issue from gprofng/src/Print.cc

I got a format-security werror on gprofng/src/Print.cc when trying to
build the master version of gdb (the compiler flag is added by the
archlinuxcn build machine).

While I could disable the flag, I think there might be a real issue
looking at the code.

The line that causes the issue is
https://github.com/bminor/binutils-gdb/blob/master/gprofng/src/Print.cc#L2616,
which uses a "dynamic" format string without any argument. AFAICT, the
fmt3 is only ever initialized in
`er_print_experiment::overview_summary` and if I read it correctly,
it's initialized to a string with no actual formatting inputs other
than a `%%`. It is used, however, twice in
`er_print_experiment::overview_value`, one given two zeros as the
arguments and one given no arguments so it looks a bit suspicious.

The git log shows now history of this file so I'm not sure what's the
intention but my best guess is

1. the `fprintf (out_file, fmt3, 0., 0.);` was meant to be using
`fmt4`. (I assume this is to avoid nan from total_value = 0), or
2. since fmt3 is actually a string that's more or less "0.0 (0.0)"
with padding, the two `0.`s passed to fmt3 are probably bogus and it
should be the same as the `fprintf (out_file, fmt3);` below

if my understanding is correct, I think in either case one can simply
avoid using fmt3 with fprintf by just removing the extra % from it and
directly write it to the output instead. (i.e.
https://gist.github.com/yuyichao/7e7cc2f240a1a6e92a1b2a9da8eb3905)
Did I miss anything?

Yichao