selftests: fix LLVM build for i386 and x86_64

Add missing cases for the i386 and x86_64 architectures when
determining the LLVM target for building kselftest.

Fixes: 795285ef

 ("selftests: Fix clang cross compilation")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>