clang: Update documentation

As of clang-5.0, things have changed a bit.  First, we cannot
automatically guess -target values as if we do not pass one with CC then
cc-option will fail.  Second, to disable movt/movw relocations the
argument has become -mno-movt.

Related to the target part, we cannot use arm-none-eabi as that ends up
being too generic of an ARM target for things like say rpi_3_32b to
work.

Signed-off-by: Tom Rini <trini@konsulko.com>