[Native] Introduce optInCacheableTargets

Create an explicit list of targets that have unstable compiler caches
in konan.properties. It allows to control all lists of cacheable targets
in a single place.

kotlin-native/konan/konan.properties

@@ -89,6 +89,12 @@ cacheableTargets.macos_arm64 = \
   ios_simulator_arm64 \
   ios_arm64
 
+# Targets that support compiler caches, but these caches
+# are not considered stable enough to be turned on by default.
+optInCacheableTargets = \
+  ios_arm64 \
+  linux_x64
+
 # Mac OS X.
 # Can be an absolute path instead of predefined value.
 llvmHome.macos_x64 = $llvm.macos_x64.dev