Update NEWS for MPX support in the dynamic linker

[Roland McGrath]

> @@ -24,6 +24,12 @@ Version 2.20
>    17050, 17058, 17061, 17062, 17069, 17075, 17078, 17079, 17084, 17086,
>    17088, 17092, 17097, 17125, 17135, 17137, 17150, 17153, 17213.
> +* Support for Intel MPX in x86 dynamic linker was added. This means Intel MPX
> +  bound registers are now saving/restoring in _dl_runtime_profile and
> +  _dl_runtime_resolve. The change revealed "Unrecognised instruction" issue
> +  in valgrind. It is known to be fixed in version 3.9 but still could be seen
> +  in older versions of valgrind and probably in other similar tools.

You need a blank line before the new item.  Two spaces between sentences.
Don't mention dynamic linker internal function names, just the user-visible
effects.  How about this?

* On x86, the dynamic linker's lazy-binding support is now compatible
  with application code using Intel MPX instructions.  (With all
  previous versions, the MPX register state could be clobbered when
  making calls into or out of a shared library.)  Note that while the
  new dynamic linker is compatible with all known x86 hardware whether
  or not it supports Intel MPX, some x86 instruction-set emulators
  might fail to handle the new instruction encodings.  This is known
  to affect Valgrind versions prior to 3.9, and might affect other
  tools that do instruction emulation.