llvm: add dependency on !BR2_TOOLCHAIN_HAS_GCC_BUG_64735

llvm: add dependency on !BR2_TOOLCHAIN_HAS_GCC_BUG_64735

LLVM uses std::shared_future, which until gcc 7.x isn't available on
architectures that don't provide lock-free atomics, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735.

Buildroot already has a BR2_TOOLCHAIN_HAS_GCC_BUG_64735 option to
handle such a case, so simply add this new dependency to LLVM. It will
make sure LLVM doesn't get built on ARMv5 platforms using a gcc older
than 7.x.

Fixes:

  http://autobuild.buildroot.net/results/ada497f6a8d20fa1a9adb2b17a138d7b726a6cdc/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: Valentin Korenblit <valentin.korenblit@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>