toolchain/toolchain-buildroot: enable glibc for all little-endian ARCs with atomic ops

Initially we had a port only for ARCv2 cores but then with a simple
change ARCompact cores got supported as well.

So we generalize from BR2_archs to BR2_arcle as we haven't tried to
get glibc working on big-endian ARCs yet.

Also we never bothered to check avaialbility of atomic instructions in
the core but in case of Glibc for ARC this is really a must, so we add
this check here.

Note in case of uClibc we may have system w/o HW atomics but:

 1. Only single-core systems are allowed
 2. Atomic instructions are emulated via arc_usr_cmpxchg syscall

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>