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Add placeholder __libcpp_relaxed_store() for when atomic builtins are not…

Description

Add placeholder __libcpp_relaxed_store() for when atomic builtins are not available.

Summary:
In rL241532, atomic_support.h was added, which provides handling of
atomic operations for libc++. When atomic builtins are not available,
it emits a warning about being unsupported, but it still provides a
number of stubs for the required functions.

However, it misses a stub for __libcpp_relaxed_store(). Add it, by
using the same implementation as for __libcpp_atomic_store().

(Note that I encountered this on arm-freebsd, which still defaults to
armv4, and does not have the runtime libcalls to support atomic
builtins. For now, I have simply disabled using them.)

Reviewers: mclow.lists, EricWF

Subscribers: theraven, cfe-commits, jroelofs, majnemer, aemerson

Differential Revision: http://reviews.llvm.org/D13051