package/gdb: remove gdb 7.12.1

package/gdb: remove gdb 7.12.1

gdb 7.12.1 was released on 21 Jan 2017 [1] and is the last version that
doesn't requires a C++11 compiler.  Since Buildroot now support three other
gdb versions (8.1, 8.2 and 8.3), reduce the number of choice by dropping
7.12.1.

Remove BR2_PACKAGE_GDB_NEEDS_CXX11 symbol since it's not useful
anymore.

[1] https://lists.gnu.org/archive/html/info-gnu/2017-01/msg00009.html

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>