CFI: Enable KCFI testing of run-pass tests

This enables KCFI-based testing for all the CFI run-pass tests in the
suite today. We can add the test header on top of in-flight CFI tests
once they land.

It also enables KCFI as a sanitizer for x86_64 and aarch64 Linux to make
this possible. The sanitizer should likely be available for all aarch64,
x86_64, and riscv targets, but that isn't critical for initial testing.

compiler/rustc_target/src/spec/targets/aarch64_unknown_linux_gnu.rs

@@ -19,6 +19,7 @@ pub fn target() -> Target {
             stack_probes: StackProbeType::Inline,
             supported_sanitizers: SanitizerSet::ADDRESS
                 | SanitizerSet::CFI
+                | SanitizerSet::KCFI
                 | SanitizerSet::LEAK
                 | SanitizerSet::MEMORY
                 | SanitizerSet::MEMTAG

compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs

@@ -10,6 +10,7 @@ pub fn target() -> Target {
     base.static_position_independent_executables = true;
     base.supported_sanitizers = SanitizerSet::ADDRESS
         | SanitizerSet::CFI
+        | SanitizerSet::KCFI
         | SanitizerSet::DATAFLOW
         | SanitizerSet::LEAK
         | SanitizerSet::MEMORY