toolchain: Add config option for atomic intrinsics

GCC has several builtin functions that implement atomic operations. Those
functions are architecture specific and may not be implemented by the
specific toolchain. In case of GCC for ARC those functions rely on
LLOCK/SCOND instructions which are optional in ARC CPU's. If ARC CPU doesn't
support those instructions but software tries to use them, then application
will be aborted with Illegal instruction exception. To avoid confusion user
should first specify that their CPU supports atomic extension, which will
allow selection of packages that use builtin atomic functions.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>