rustc: implement support for riscv32im_risc0_zkvm_elf

This also adds changes in the rust test suite in order to get a few of them to
pass.

Co-authored-by: Frank Laub <flaub@risc0.com>
Co-authored-by: Urgau <3616612+Urgau@users.noreply.github.com>

library/test/src/console.rs

@@ -323,7 +323,8 @@ pub fn run_tests_console(opts: &TestOpts, tests: Vec<TestDescAndFn>) -> io::Resu
     // Prevent the usage of `Instant` in some cases:
     // - It's currently not supported for wasm targets.
     // - We disable it for miri because it's not available when isolation is enabled.
-    let is_instant_supported = !cfg!(target_family = "wasm") && !cfg!(miri);
+    let is_instant_supported =
+        !cfg!(target_family = "wasm") && !cfg!(target_os = "zkvm") && !cfg!(miri);
 
     let start_time = is_instant_supported.then(Instant::now);
     run_tests(opts, tests, |x| on_test_event(&x, &mut st, &mut *out))?;