toolchain/toolchain-common.in: fix definition of BR2_TOOLCHAIN_HAS_GCC_BUG_64735

The rework done on commit
accba02a ("toolchain: add option for
toolchains affected by GCC PR libstdc++/64735") by me was wrong. The
BR2_TOOLCHAIN_HAS_GCC_BUG_64735 option should be enabled when the bug is
present in the toolchain, not the opposite.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>