Fix: commonizable PlatformMutex on native (#329) - #330

Closes #329

• Cause: PlatformMutex.native.kt (HandleTable mutated without the GIL via _pm_release (suspected race) #117, Binder: HandleTable owns a mutex (#117) #302) called pthread_mutex_init, which is not in the POSIX surface Kotlin/Native commonizes across Apple and Linux. :python-multiplatform:compileNativeMainKotlinMetadata, which publishAllToMavenLocal runs, therefore failed. The per-target compiles passed, so the regression went unnoticed.
• Fix: a non-recursive CAS spin lock on kotlin.concurrent.atomics.AtomicInt, the same common atomics PyType uses. PlatformMutex's contract keeps critical sections to a few array and counter operations and never calls Python, so spinning is bounded and cannot hold up a stop-the-world pause.
• Red: with the old file, compileNativeMainKotlinMetadata fails with 'Unresolved reference pthread_mutex_init'.
• Green: compileNativeMainKotlinMetadata, compileKotlinIosSimulatorArm64 and compileKotlinAndroidNativeArm64 pass. iosSimulatorArm64Test (HandleTable and GilReleaseNativeTest) passes 16/16.