clang-cross: trim the recipes

clang-cross: trim the recipes

As a single clang-native build is a cross-compiler for all targets, this
recipe should simply create target-prefixed symlinks.

As such, it doesn't need to pull in the common clang include files, or
set TOOLCHAIN.

Unsetting PACKAGES isn't needed as cross inherits nopackages.

The sysroot preprocessing is used to stage bindir, but cross recipes
already stage this directory.

There doesn't appear to be a need to copy clang, so these are now linked
too.

SSTATE_SCAN_FILES only works on actual files not symlinks, so setting it
is pointless.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>