[AMDGPU] Place global constructors in .init_array and .fini_array

For the GPU, we emit external kernels that call the initializers and
constructors, however if we had a persistent kernel like in the `_start`
kernel for the `libc` project, we could initialize the standard way of
calling constructors. This patch adds new global variables containing
pointers to the constructors to be called. If these are placed in the
`.init_array` and `.fini_array` sections, then the backend will handle
them specially. The linker will then provide the `__init_array_` and
`__fini_array_` sections to traverse them. An implementation would look
like this.

```
extern uintptr_t __init_array_start[];
extern uintptr_t __init_array_end[];
extern uintptr_t __fini_array_start[];
extern uintptr_t __fini_array_end[];

using InitCallback = void(int, char **, char **);
using FiniCallback = void(void);

extern "C" [[gnu::visibility("protected"), clang::amdgpu_kernel]] void
_start(int argc, char **argv, char **envp) {
  uint64_t init_array_size = __init_array_end - __init_array_start;
  for (uint64_t i = 0; i < init_array_size; ++i)
    reinterpret_cast<InitCallback *>(__init_array_start[i])(argc, argv, env);
  uint64_t fini_array_size = __fini_array_end - __fini_array_start;
  for (uint64_t i = 0; i < fini_array_size; ++i)
    reinterpret_cast<FiniCallback *>(__fini_array_start[i])();
}
```

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D149340