[libc] Warn on use of global constructors in libc

Clang supports the `-Wglobal-constructors` flag which will indicate if a
global constructor is being used. The current goal in `libc` is to make
the constructors `constexpr` to prevent this from happening with
straight construction. However, there are many other cases where we can
emit a constructor that this won't catch. This should give warning if
someone accidentally introduces a global constructor.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D155721

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -38,6 +38,7 @@ function(_get_common_compile_options output_var flags)
       list(APPEND compile_options "-Wnonportable-system-include-path")
       list(APPEND compile_options "-Wstrict-prototypes")
       list(APPEND compile_options "-Wthread-safety")
+      list(APPEND compile_options "-Wglobal-constructors")
     endif()
     if(ADD_FMA_FLAG)
       if(LIBC_TARGET_ARCHITECTURE_IS_X86)