Fix build on linux clang targets

In CheckAtomic.cmake we include is_lock_free because these routines
don't seem to be included in the compiler's "simple atomics". This
triggers a failure in clang toolchain, forcing it to link libatomic.

Resulting in error: ld.lld: error: undefined symbol: __atomic_is_lock_free

CCBUG:444247
CCBUG:444547
(cherry picked from commit 0b755bea)