```
AppProperties app;
```
is marked as a weak symbol in header now. One can just use `&app !=
nullptr` to check if `app` is defined. There is no need to define it for
overlay mode.
For some reasons, we are using `-fpie`
(libc/cmake/modules/LLVMLibCObjectRules.cmake:31) without supporting it.
According to @lntue, some of the hermetic tests are broken without
proper PIE support. This patch implements basic relocations support for
PIE.
* 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)
```
