doc: clang: Update and correct support notes

At this point, clang can be used on both 32bit and 64bit targets without
issue. Make note of logic we have that will inform clang of the
architecture to build for.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
1 file changed