* 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 mostly renames files so it better reflects the function they declare.
Reviewed By: michaelrj
Differential Revision: https://reviews.llvm.org/D155607
Summary:
The changes in D146184 made the integration tests use the inhereted
dependencies from the startup code like a normal target. For the AArch64
target this resulted in the threads depenency not being pulled in
because it was not present in the original code.
