[libc] major refactor of startup library (#76092)

* 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) 
```

1 files changed, 6 insertions, 0 deletions

diff --git a/libc/cmake/modules/LLVMLibCTestRules.cmake b/libc/cmake/modules/LLVMLibCTestRules.cmake
index 51d484b..b69839a 100644
--- a/libc/cmake/modules/LLVMLibCTestRules.cmake
+++ b/libc/cmake/modules/LLVMLibCTestRules.cmake
@@ -498,6 +498,9 @@ function(add_integration_test test_name)
       libc.src.string.memcpy
       libc.src.string.memmove
       libc.src.string.memset
+      # __stack_chk_fail should always be included to allow building libc with
+      # stack protector.
+      libc.src.compiler.__stack_chk_fail
   )
   list(REMOVE_DUPLICATES fq_deps_list)
 
@@ -665,6 +668,9 @@ function(add_libc_hermetic_test test_name)
       libc.src.string.memmove
       libc.src.string.memset
       libc.src.__support.StringUtil.error_to_string
+      # __stack_chk_fail should always be included to allow building libc with
+      # stack protector.
+      libc.src.compiler.__stack_chk_fail
   )
 
   if(TARGET libc.src.time.clock)