Merge pull request #2538 from ludovic-henry/fix-sgen-38012

[sgen] Fix register scanning on ARM

This bug would trigger a use after sweep in System.Threading.Tasks.Task.FinishContinuations:3621.

The issue would arise on ARM, as `MonoContext` and `ARCH_NUM_REGS` are defined as followed :

```
typedef struct {
  mgreg_t pc;
  mgreg_t regs [16];
  double fregs [16];
  mgreg_t cpsr;
} MonoContext;

\#define ARCH_NUM_REGS 14
```

As you can see, the MonoContext structure is bigger than 14 words, and it does not even covers the last 3 values of `regs`. By using pointer arithmetic, we ensure that we do not miss some parts of the `MonoContext` structure.

The observed behaviour in System.Threading.Tasks.Task.FinishContinuations would be that `continuationObject` would not be marked, and thus freed; `continuationObject` would still contains the pointer to the old location, meaning it wouldn't be null. We would then try to check its type via a call to the `as` operator. This would call the `isinst` IL opcode which would load `continuationObject->vtable->klass->supertypes [...]`, which would trigger a segfault, as `supertypes` would be null.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=38012