RFA/doco: Various changes related to GMP and fixed point type support

[Joel Brobecker]

Hello,

This patch series only updates our NEWS and README files, following
some changes I pushed recently (thanks to Pedro for reminding me
of this).

  - [RFA/doco 1/4] gdb/NEWS: Document that building GDB now requires GMP
  - [RFA/doco 2/4] gdb/NEWS: Document that GDB now supports DWARF-based
  - [RFA/doco 3/4] gdb/README: Document the --with-libgmp-prefix
  - [RFA/doco 4/4] gdb/README: Fix the URL of the MPFR website (now

OK to push to master?

Thank you!
-- 
Joel