CMake: refactor architecture-specific build system parts.

Two user-visible changes were made:
* `-DUSE_RUST` is replaced with `-DBUILD_RUST`, by analogy with
  `-DBUILD_PYTHON`
* `-DCOVERAGE` was removed as it doesn't work with either modern GCC
  or Clang

.github/ci/build_himbaechel.sh

@@ -8,7 +8,7 @@ function build_nextpnr {
     mkdir build
     pushd build
     cmake .. -DARCH=himbaechel -DHIMBAECHEL_UARCH=example -DHIMBAECHEL_EXAMPLE_DEVICES=example
-    make nextpnr-himbaechel chipdb-himbaechel-example -j`nproc`
+    make nextpnr-himbaechel -j`nproc`
     popd
 }