Link the concurrency runtime against libatomic on Linux.

We mostly get away without this because we're fairly disciplined
about using constant memory orderings, and apparently that's
usually good enough to get inline accesses and avoid needing to
link atomic.  However, we have a few places with the task status
atomic that use a non-constant load ordering with load and
compare_exchange_weak, and my recent change to make that atomic
a double-word was apparently sufficient on some (but not all)
Linux distributions to get the compiler to call the runtime
function.  Regardless, we shouldn't be playing around in the
margins here: Linux requires us to link libatomic, so we should.

Author: rjmccall

stdlib/public/Concurrency/CMakeLists.txt

@@ -29,6 +29,16 @@ if(SWIFT_CONCURRENCY_USES_DISPATCH)
   endif()
 endif()
 
+# Linux requires us to link an atomic library to use atomics.
+# Frustratingly, in many cases this isn't necessary because the
+# sequence is inlined, but we have some code that's just subtle
+# enough to turn into runtime calls.
+if(SWIFT_HOST_VARIANT STREQUAL "Linux")
+  if(SWIFT_HOST_VARIANT_ARCH MATCHES "armv6|armv7|i686")
+    list(APPEND swift_concurrency_link_libraries
+      atomic)
+  endif()
+endif()
 
 add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
   ../CompatibilityOverride/CompatibilityOverride.cpp