internal/buildcfg: expand spinbitmutex platforms

Go 1.24 included the spinbitmutex GOEXPERIMENT for several popular
architectures, based on their native support an atomic primitive (8-bit
exchange) that aided its efficient implementation.

Move towards making the new mutex implementation permanent, so it fully
replaces the two previous (sema- and futex-based "tristate")
implementations.

For #68578

Change-Id: I888a73959df42eb0ec53875309c446675af8f09d
Reviewed-on: https://go-review.googlesource.com/c/go/+/658455
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Rhys Hiltner <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: rhysh

src/internal/buildcfg/exp.go

@@ -67,11 +67,7 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		regabiSupported = true
 	}
 
-	var haveXchg8 bool
-	switch goarch {
-	case "386", "amd64", "arm", "arm64", "ppc64le", "ppc64":
-		haveXchg8 = true
-	}
+	haveThreads := goarch != "wasm"
 
 	// Older versions (anything before V16) of dsymutil don't handle
 	// the .debug_rnglists section in DWARF5. See
@@ -89,7 +85,7 @@ func ParseGOEXPERIMENT(goos, goarch, goexp string) (*ExperimentFlags, error) {
 		RegabiArgs:      regabiSupported,
 		AliasTypeParams: true,
 		SwissMap:        true,
-		SpinbitMutex:    haveXchg8,
+		SpinbitMutex:    haveThreads,
 		SyncHashTrieMap: true,
 		Dwarf5:          dwarf5Supported,
 	}