[ELF] Fix incorrect sorting of .init_array / .fini_array sections.

The main problem is in the predicate passed to the `std::stable_sort()`.
This predicate always returns false if **both** section's names do not
start with `.init_array` or `.fini_array` prefixes. In short, it does not
define a strict weak orderng. Suppose we have the following sections:

  .A .init_array.1 .init_array.2

The predicate states that:

  not .init_array.1 < .A
  not .A < .init_array.2
  but .init_array.1 < .init_array.2 !!!

The second problem is that `.init_array` section without number should
go last in the list. Not it has the lowest priority '0' and goes first.

The patch fixes both of the problems.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209875 91177308-0d34-0410-b5e6-96231b3b80d8