clang does not automatically link `atomic` builtins

[huixie90]

On Linux (and others?), the __atomic_store (and other atomic) builtin doesn't work out of the box.
User will see a linker error

undefined reference to `__atomic_store'

If the system has gcc installed, the user could add -latomic. However, on systems without gcc installed, the user would have to find the correct compiler-rt library that needs to be linked, which is not user friendly. clang should just atomically link to the compiler-rt when the builtin needs to be linked

Reproducer:

cat <<EOF | clang++ -xc++ -
struct Large {
    int value[100];
};

struct GCC {
    Large value;
};

void gcc(GCC& a, Large& val, int memory_order) {
    __atomic_store(&a.value, &val, memory_order);
}


struct C11 {
    _Atomic(Large) value;
};

void c11(C11& a, Large& val, int memory_order) {
    __c11_atomic_store(&a.value, val, memory_order);
}

int main() { }
EOF

Godbolt: https://godbolt.org/z/f4jchEvxv

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Hui (huixie90)

On Linux (and others?), the `__atomic_store` (and other atomic) builtin doesn't work out of the box. User will see a linker error ``` undefined reference to `__atomic_store' ```

If the system has gcc installed, the user could add -latomic. However, on systems without gcc installed, the user would have to find the correct compiler-rt library that needs to be linked, which is not user friendly. clang should just atomically link to the compiler-rt when the builtin needs to be linked

Reproducer:

cat <<EOF | clang++ -xc++ -
struct Large {
    int value[100];
};

struct GCC {
    Large value;
};

void gcc(GCC& a, Large& val, int memory_order) {
    __atomic_store(&a.value, &val, memory_order);
}


struct C11 {
    _Atomic(Large) value;
};

void c11(C11& a, Large& val, int memory_order) {
    __c11_atomic_store(&a.value, val, memory_order);
}

int main() { }
EOF

Godbolt: https://godbolt.org/z/f4jchEvxv