[libc] Add the 'cpp.new' as a dependency on `atexit`

[jhuber6]

Summary

The atexit function depends on the implementations in CPP/new.h but it
is not listed as a dependency. This causes the GPU build to not include
it in the libcgpu.a file and prevents us from using the startup code
externally. Simply add it.

libc/src/stdlib/CMakeLists.txt

add_entrypoint_object(
   HDRS
     atexit.h
   CXX_STANDARD
     20 # For constinit of the atexit callback list.
   DEPENDS
     libc.src.__support.fixedvector
     libc.src.__support.blockstore
     libc.src.__support.threads.mutex
+    libc.src.__support.CPP.new
 )
 
 add_entrypoint_object(
   exit
   SRCS
     exit.cpp
   HDRS
     exit.h