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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186394 91177308-0d34-0410-b5e6-96231b3b80d8