[System.Runtime.Caching] Added Mono config variables.
Variables __MonoEmulateOneCPU and __MonoTimerPeriod are used by some tests.
[System.Runtime.Caching] Removed UnsafeNativeMethods calls on Mono.
 - MEMORYSTATUSEX usages were replaced with performance counters
   "Total Physical Memory and "Available Physical Memory"

 - SafeRegistryHandle.ReleaseHandle is only called on code that is ignored on Mono.

 - Mono is using AppDomain.FriendlyName directly instead of UnsafeNativeMethods.GetModuleFileName.