buildroot

gdb: unbreak 7.12 target gdb/gdbserver build without C++ support

gdb 7.12+ by default builds with a C++ compiler, which naturally doesn't
work when we don't have C++ support in the toolchain.

Fix it by passing --disable-build-with-cxx for such setups.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>