Provide an acceptable implementation of __cxa_guard_acquire and friends

The following code fails to compile with linker errors due to default (libstdc++) implementation of __cxa_guard_acquire(), __cxa_guard_release() which pull in large link dependencies. The attached patch fixes this by providing acceptable defaults. Note that if a multitasking OS is implemented for the spark-core, these implementations will need to be revisited.

Compiles OK:
```
class A {
    uint8_t a;
public:
    A() : a(0) {}
    void beA() {++a;}
};

static A a;

void setup() {
    a.beA();
}

void loop() {
}
```

While 

```
class A {
    uint8_t a;
public:
    A() : a(0) {}
    void beA() {++a;}
};

void setup() {
    static A a;
    a.beA();
}

void loop() {
}
```

fails with:

```
/opt/gcc_arm/bin/../lib/gcc/arm-none-eabi/4.7.4/../../../../arm-none-eabi/bin/ld: 68c052164109e4a9717a31d585718ed28c0a724531c5b4b69b8f62aced7c.elf section `.text' will not fit in region `FLASH'
```