[lld][ELF] Add some release notes

* Relax MIPS ``jalr``and ``jr`` instructions marked by the ``R_MIPS_JALR``

  relocation.

* For certain "undefined symbol" errors, a definition with a close spelling will be suggested.

  (`D67039 <https://reviews.llvm.org/D67039>`_)

* ``extern "C"`` is suggested if an undefined reference is mangled(unmangled) while there

  is a likely unmangled(mangled) definition.

  (`D69592 <https://reviews.llvm.org/D69592>`_ `D69650 <https://reviews.llvm.org/D69650>`_)

* New ``-z noseparate-code``, ``-z separate-code`` and ``-z separate-loadable-segments``.

  ``-z noseparate-code`` is the default, which can reduce sizes of linked binaries by up to

  3 times maxpagesize.

  (`D64903 <https://reviews.llvm.org/D64903>`_ `D67481 <https://reviews.llvm.org/D67481>`_)

* ``-z force-bti`` and ``-z pac-plt`` are added for AArch64 Branch Target Identification and Pointer Authentication.

  (`D62609 <https://reviews.llvm.org/D62609>`_)

* ``--fix-cortex-a8`` is added to fix erratum 657417.

  (`D67284 <https://reviews.llvm.org/D67284>`_)

* ``-z force-ibt`` and ``-z shstk`` are added for Intel Control-flow Enforcement Technology.

  (`D59780 <https://reviews.llvm.org/D59780>`_)

* ``PT_GNU_PROPERTY`` is added to help loaders locate the ``.note.gnu.property`` section.

  It may be used by a future Linux kernel.

  (`D70961 <https://reviews.llvm.org/D70961>`_)

* For ``--compress-debug-sections=zlib``, ``-O0`` and ``-O1`` enable compression level 1

  while ``-O2`` enables compression level 6. ``-O1`` (default) is faster than before.

  (`D70658 <https://reviews.llvm.org/D70658>`_)

* Range extension thunks with addends are implemented for AArch64, PowerPC32 and PowerPC64.

  (`D70637 <https://reviews.llvm.org/D70637>`_ `D70937 <https://reviews.llvm.org/D70937>`_

  `D73424 <https://reviews.llvm.org/D73424>`_)

* ``R_RISCV_ALIGN`` will be errored because linker relaxation for RISC-V is not supported.

  Pass ``-mno-relax`` to disable ``R_RISCV_ALIGN``.

  (`D71820 <https://reviews.llvm.org/D71820>`_)

* The ARM port will no longer insert interworking thunks for non STT_FUNC symbols.

  (`D73474 <https://reviews.llvm.org/D73474>`_)

* The quality of PowerPC32 port has been greatly improved (canonical PLT, copy

  relocations, non-preemptible IFUNC, range extension thunks with addends).

  It can link FreeBSD 13.0 userland.

* The PowerPC64 port supports non-preemptible IFUNC.

  (`D71509 <https://reviews.llvm.org/D71509>`_)

* lld creates a RO PT_LOAD and a RX PT_LOAD without a linker script.

  lld creates a unified RX PT_LOAD with a linker script.

  A future release will eliminate this difference and use a RO PT_LOAD and a RX PT_LOAD by default.

  The linker script case will require ``--no-rosegment`` to restore the current behavior.

* GNU style compressed debug sections ``.zdebug`` (obsoleted by ``SHF_COMPRESSED``)

  are supported for input files, but not for the output.

  A future release may drop ``.zdebug`` support.

Breaking changes

----------------

* Reduced size of linked MIPS binaries.

* ``-Ttext=$base`` (base is usually 0) is no longer supported.

  If PT_PHDR is needed, use ``--image-base=$base`` instead.

  If PT_PHDR is not needed, use a linker script with `.text 0 : { *(.text*) }` as the first

  output section description.

  See https://bugs.llvm.org/show_bug.cgi?id=44715 for more information.

  (`D67325 <https://reviews.llvm.org/D67325>`_)

* ``-Ttext-segment`` is no longer supported. Its meaning was different from GNU ld's and

  could cause subtle bugs.

  (`D70468 <https://reviews.llvm.org/D70468>`_)

COFF Improvements

-----------------