doc: move Clang documentation to HTML

* Move README.clang to doc/build/clang.rst and reformat as
  reStructeredText.
* Indicate that -ffixed-r9 and -ffixed-x18 are used to reserve registers
  for gd.
* Minor editing.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>