Startup code

[mchokshi091101]

• Rebased Startup Code PR
• Added _Exit() function in crt1.cc

[konstantinschwarz]

Until we update to the most recent libc build system, we need to add the following to fix the build & install targets of the startup files:

diff --git a/libc/lib/CMakeLists.txt b/libc/lib/CMakeLists.txt
index 652ebc9ffb46..cffe47dbf931 100644
--- a/libc/lib/CMakeLists.txt
+++ b/libc/lib/CMakeLists.txt
@@ -41,6 +41,9 @@ foreach(archive IN ZIP_LISTS
   # with custom targets and in older version of cmake (~pre-3.20, apparently)
   # there is not good support for describing install dependencies on custom targets.
   add_dependencies(${archive_1} libc-headers)
+  if(TARGET libc-startup)
+    add_dependencies(${archive_1} libc-startup)
+  endif()
 
   set_target_properties(
     ${archive_1}

[konstantinschwarz]

Please squash the commits during rebase&merge