Rename `CI_HOST_ARCH` and `CI_HOST_SYSTEM`, to `CI_BUILD_ARCH` and
`CI_BUILD_SYSTEM`, following the nomenclature used by GNU Autotools.
Unfortunately, the word "host" has confusingly opposite meanings in
CMake (and Bazel, etc.) vs. Autotools (and Meson, etc.)
Remove `CI_TARGET_TRIPLET` and `CI_TARGET_ABI` (for now).
Introduce the function `ci_expr` as a fast and easy equivalent of
`expr >/dev/null`.
Rephrase the assertions using an implementation pattern that is more
expressive, yet (arguably) just as readable. Remove `ci_assert`.
Modify the main functions to display more useful information in case
of usage error.
