cranelift-native flags detection: fix flags on SSE2-only systems.

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.

cranelift/codegen/src/isa/x64/mod.rs

@@ -182,6 +182,12 @@ fn isa_constructor(
 
  // Check for compatibility between flags and ISA level
  // requested. In particular, SIMD support requires SSE4.2.
+ println!(
+ "simd = {} sse3 = {} sse42 = {}",
+ shared_flags.enable_simd(),
+ isa_flags.has_sse3(),
+ isa_flags.has_sse42()
+ );
  if shared_flags.enable_simd() {
  if !isa_flags.has_sse3()
  || !isa_flags.has_ssse3()

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();
  }