toolchain/toolchain-common.in: adjust BR2_TOOLCHAIN_HAS_GCC_BUG_64735 for GCC 7

As GCC 7 is now available in Buildroot, update the definition for
BR2_TOOLCHAIN_HAS_GCC_BUG_64735 as the bug #64735 is fixed in GCC 7.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>