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>
diff --git a/doc/build/index.rst b/doc/build/index.rst
index e4e3411..e0072af 100644
--- a/doc/build/index.rst
+++ b/doc/build/index.rst
@@ -6,4 +6,5 @@
 .. toctree::
    :maxdepth: 2
 
+   clang
    tools