[Bug bootstrap/87741] Don't build readline/libreadline.a in GDB, when --with-system-readline is supplied

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87741

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Patches should be sent to gcc-patches@ after reading
https://gcc.gnu.org/contribute.html .