Work around clang emitting libcalls to atomic_add_fetch() and friends
in libc++, on ARM_ARCH < 6. Additionally, supply the missing stub
__libcpp_relaxed_store(), as proposed in http://reviews.llvm.org/D13051
NOTE: this needs to be fixed properly later on, by supplying library
functions implementing atomic operations for arm < v6. We should
probably take those from sys/arm/arm/stdatomic.c, and stuff them into
either libgcc or compiler-rt.