ARM: implement low-level intrinsics for the atomic exclusive operations.

This adds three overloaded intrinsics to Clang:
    T __builtin_arm_ldrex(const volatile T *addr)
    int __builtin_arm_strex(T val, volatile T *addr)
    void __builtin_arm_clrex()

The intent is that these do what users would expect when given most sensible
types. Currently, "sensible" translates to ints, floats and pointers.

llvm-svn: 186394