cranelift-native flags detection: fix flags on SSE2-only systems. (#4231)

In #4224 we saw that an SSE2-only x86-64 system somehow was still
 detecting SSE3/SSSE3/SSE4.1/SSE4.2. It turns out that we enabled these
 in the baseline `Flags` in #3816, because without that, a ton of other
 things break: default flags no longer produce a compiler backend that
 works with default Wasmtime settings. However the logic to set them
 when detected (via `CPUID`-using feature-test macros) only does an "if
 detected then set bit" step per feature; the bits are never *cleared*.
 This PR fixes that.

Author: cfallin

cranelift/native/src/lib.rs

@@ -58,6 +58,19 @@ pub fn builder_with_options(infer_native_flags: bool) -> Result<isa::Builder, &'
             return Ok(isa_builder);
         }
 
+        // These are temporarily enabled by default (see #3810 for
+        // more) so that a default-constructed `Flags` can work with
+        // default Wasmtime features. Otherwise, the user must
+        // explicitly use native flags or turn these on when on x86-64
+        // platforms to avoid a configuration panic. In order for the
+        // "enable if detected" logic below to work, we must turn them
+        // *off* (differing from the default) and then re-enable below
+        // if present.
+        isa_builder.set("has_sse3", "false").unwrap();
+        isa_builder.set("has_ssse3", "false").unwrap();
+        isa_builder.set("has_sse41", "false").unwrap();
+        isa_builder.set("has_sse42", "false").unwrap();
+
         if std::is_x86_feature_detected!("sse3") {
             isa_builder.enable("has_sse3").unwrap();
         }