selftests: fix LLVM build for i386 and x86_64

selftests: fix LLVM build for i386 and x86_64

BugLink: https://bugs.launchpad.net/bugs/2016876
[ Upstream commit 624c60f326c6e5a80b008e8a5c7feffe8c27dc72 ]

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

Fixes: 795285ef2425 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>