[KernelAddressSanitizer] Make globals constructors compatible with kernel [v2]

[ v1 was reverted by c6ec352a due to
  modpost failing; v2 fixes this. More info:
  https://github.com/ClangBuiltLinux/linux/issues/1045#issuecomment-640381783 ]

This makes -fsanitize=kernel-address emit the correct globals
constructors for the kernel. We had to do the following:

* Disable generation of constructors that rely on linker features such
  as dead-global elimination.

* Only instrument globals *not* in explicit sections. The kernel uses
  sections for special globals, which we should not touch.

* Do not instrument globals that are prefixed with "__" nor that are
  aliased by a symbol that is prefixed with "__". For example, modpost
  relies on specially named aliases to find globals and checks their
  contents. Unfortunately modpost relies on size stored as ELF debug info
  and any padding of globals currently causes the debug info to cause size
  reported to be *with* redzone which throws modpost off.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=203493

Tested:
* With 'clang/test/CodeGen/asan-globals.cpp'.

* With test_kasan.ko, we can see:

  	BUG: KASAN: global-out-of-bounds in kasan_global_oob+0xb3/0xba [test_kasan]

* allyesconfig, allmodconfig (x86_64)

Reviewed By: glider

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D81390