toolchain: drop the now-unused old BR2_TOOLCHAIN_HAS_ATOMIC_INTRINSICS

It's now been replaced with BR2_ARCH_HAS_ATOMICS, annd all packages have
been changed to use that instead.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>