Summary:
This patch implements 'getenv'. I was torn on how to implement this,
since realistically we only have access to this environment pointer in
the "loader" interface. An alternative would be to use an RPC call every
time, but I think that's overkill for what this will be used for. A
better solution is just to emit a common `DataEnvironment` that contains
all of the host visible resources to initialize. Right now this is the
`env_ptr`, `clock_freq`, and `rpc_client`.

I did this by making the `app.h` interface that Linux uses more general,
could possibly move that into a separate patch, but I figured it's
easier to see with the usage.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

This PR fix several build errors on aarch64 targets when building with
gcc:
- uninitialized values leading to `Werrors`
- undefined builtin functions
- glibc header pollution

* separate initialization routines into _start and do_start for all
architectures.
* lift do_start as a separate object library to avoid code duplication.
* (addtionally) address the problem of building hermetic libc with
-fstack-pointer-*

The `crt1.o` is now a merged result of three components:

```
___
  |___ x86_64
  |      |_______ start.cpp.o    <- _start (loads process initial stack and aligns stack pointer)
  |      |_______ tls.cpp.o      <- init_tls, cleanup_tls, set_thread_pointer (TLS related routines) 
  |___ do_start.cpp.o            <- do_start (sets up global variables and invokes the main function) 
```

This patch lifts aux vector related definitions to app.h. Because
startup's refactoring is in progress, this patch still contains
duplicated changes. This problem will be addressed very soon in an
incoming patch.

If a function is declared with stack-protector, the compiler may try to
load the TLS. However, inside certain runtime functions, TLS may not be
available. This patch disables stack protectors for such routines to fix
the problem.

Closes #74487.

This PR unifies the startup library's code style with the rest of libc.