Follow-up r364753 by only using arm's stdatomic.c implementation, as it

already covers the functions in compiler-rt's atomic.c, leading to
conflicts when linking.

PR:		230888
MFC after:	3 days
X-MFC-With:	r364753

lib/libcompiler_rt/Makefile.inc

@@ -124,7 +124,8 @@ SRCF+=		umodti3
 
 # Enable compiler-rt's atomic implementation only for clang, as it uses clang
 # specific builtins, and gcc packages usually come with their own libatomic.
-.if "${COMPILER_TYPE}" == "clang"
+# Exclude arm which has its own implementations of atomic functions, below.
+.if "${COMPILER_TYPE}" == "clang" && ${MACHINE_CPUARCH} != "arm"
 SRCF+=		atomic
 .endif