Properly check for errors and rename the function since we are checking if we are
running under gdb, not making it run under gdb. Previously we were
passing uninitialized data to basename() due to not properly adding the
nul termination.

==22293== Conditional jump or move depends on uninitialised value(s)
==22293==    at 0x4C306D0: rindex (vg_replace_strmem.c:199)
==22293==    by 0x4EC55DD: basename (in /usr/lib64/libc-2.27.so)
==22293==    by 0x400744: running_under_gdb (in /tmp/a)

There's another problem with this function that it doesn't detect when
we are running gdb from a toolchain using a toolchain triplet, but
that's left for another patch.

v2: remove the fix for repeating the argument on readlink() since that
    landed in another patch

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>