kernel: Clear pages allocated with SetHeapSize (#2776)

* kernel: Clear pages allocated with SetHeapSize

Before this commit, all new pages allocated by SetHeapSize were not
cleared by the kernel.

This would cause undefined data to be pass to the userland and possibly
resulting in weird memory corruption.

This commit also add support for custom fill heap and ipc value (that is also
supported by the official kernel)

* Remove dots at the end of KPageTableBase.MapPages new documentation

* Remove unused _stackFillValue

Ryujinx.HLE/HOS/Kernel/Memory/MemoryFillValue.cs

@@ -0,0 +1,10 @@
+namespace Ryujinx.HLE.HOS.Kernel.Memory
+{
+    enum MemoryFillValue : byte
+    {
+        Zero = 0,
+        Stack = 0x58,
+        Ipc = 0x59,
+        Heap = 0x5A,
+    }
+}