When build Xen target with Clang, the linker reports failure.
This patch adds the related info in the documentation as a known issue
and gives details for how to dismiss the building failure with Clang.
Signed-off-by: Leo Yan <leo.yan@linaro.org>
* 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>