[ELF] Place .note in the first page to ensure they are available in core files

Summary:
Other large sections (e.g. .rela.dyn .dynstr) may push .note.* off the
first page. They won't be available in core files if RLIMIT_CORE is
limited.

This patch gives priority to alloctable SHT_NOTE sections so that they
are assuredly in the first page and will be available in core files.
They are small and contain important information (e.g. .note.gnu.build-id
identifies the origin of the core, .note.tag stores NT_FREEBSD_ABI_TAG).

Note: gold Output_section_order has a similar rule:

  // Loadable read-only note sections come next so that the PT_NOTE
  // segment is on the first page of the executable.
  ORDER_RO_NOTE,

Reviewers: ruiu, pcc, espindola

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D55800

llvm-svn: 349524