toolchain: add option for toolchains affected by GCC PR libstdc++/64735

toolchain: add option for toolchains affected by GCC PR libstdc++/64735

exception_ptr, nested_exception, and future from libstdc++ are not
available for architectures not supporting always lock-free atomic ints
before GCC 7.

Bug report:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

Fix available starting from GCC 7 (not yet released):
https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=244051

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
[Thomas:
 - directly define the value where BR2_TOOLCHAIN_HAS_GCC_BUG_64735
   rather than having additional patches touching affected architectures
   Config.in files
 - add a better comment above the Config.in option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>