selftests: Fix clang cross compilation

Unlike GCC clang uses a single compiler image to support multiple target
architectures meaning that we can't simply rely on CROSS_COMPILE to select
the output architecture. Instead we must pass --target to the compiler to
tell it what to output, kselftest was not doing this so cross compilation
of kselftest using clang resulted in kselftest being built for the host
architecture.

More work is required to fix tests using custom rules but this gets the
bulk of things building.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>