uclibc: arc: Disable usage of atomic ops if hardware has no LLSC

uclibc: arc: Disable usage of atomic ops if hardware has no LLSC

Since uClibc-ng v1.0.20 we can force libc to either use llock/scond
instructions for atomic ops or if those are missing use kernel-assisted
emulation. By default atomics are used so we disable that feature in
libc if we build for a target without LLSC.

This allows us to build and use NPTL on ARC750 for example which was not
possible before.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>