[Mips] Implement .{ctors,dtors}.<priority> sections ordering.

Arrange .ctors/.dtors sections in the following order:
  .ctors from crtbegin.o or crtbegin?.o
  .ctors from regular object files
  .ctors.* (sorted) from regular object files
  .ctors from crtend.o or crtend?.o

This order is specific for MIPS traget. For example, on X86
the .ctors.* sections are merged into the .init_array section.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209987 91177308-0d34-0410-b5e6-96231b3b80d8