Make multithreadingSupported a property with a getter (#3130)

* Make multithreadingSupported a property with a getter

This change fixes an issue with the initialization order observed when
lazy initialization is disabled (-Xir-property-lazy-initialization=
disabled compiler flag) and the new MM is used.

The problem is the following: the initialization of DefaultDelay
happens before the initialization of multithreadingSupported. Thus
the initialization of DefaultDelay gets an uninitialized value
of multithreadingSupported and behaves as if the old MM is used.

Related issue: #KT-50491.

kotlinx-coroutines-core/native/src/internal/Concurrent.kt

@@ -32,6 +32,7 @@ internal open class SuppressSupportingThrowableImpl : Throwable() {
     }
 }
 
-@SharedImmutable
+// getter instead of a property due to the bug in the initialization dependencies tracking with '-Xir-property-lazy-initialization=disabled' that Ktor uses 
 @OptIn(ExperimentalStdlibApi::class)
-internal val multithreadingSupported: Boolean = kotlin.native.isExperimentalMM()
+internal val multithreadingSupported: Boolean
+    get() = kotlin.native.isExperimentalMM()