[libc] Ignore 'errno' on the GPU and support 'atoi'

The 'errno' value is most likely not useful on the GPU and it prevents
us from providing certain functions on the GPU that depend on it, like
`atoi`. This patch makes the necessary changes to support `errno` by
simple replacing it with a consumer class.

Supporting `errno` on the GPU is possible in some aspects. The first
approach would be to use a buffer of shared memory that has enough space
for all threads. Another option would be to change code generation to
support `thread_local` using `address_space(5)` memory allocated at
kernel launch. The former could look like the following, which could be
implemented in a later patch:

```
template <typename T>
using SharedBuffer = T[gpu::MAX_THREADS] [[clang::address_space(3)]];
template <typename T> struct ErrnoSetter {
  constexpr ErrnoSetter(SharedBuffer<T> &storage) : storage(storage) {}
  SharedBuffer<T> &storage;
  void operator=(const T &val) { storage[gpu::get_thread_id()] = val; }
};

static SharedBuffer<int> thread_local_buffer [[clang::loader_uninitialized]];
ErrnoSetter<int> __llvmlibc_internal_errno(thread_local_buffer);
```

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D149107