MemArena: Rename GrabLowMemSpace to GrabSHMSegment

This code originally tried to map the "low space" for the Gamecube's
memory layout, but since has expanded to mapping all of the easily
mappable memory on the system. Change the name to "GrabSHMSegment" to
indicate that we're looking for a shared memory segment we can map into
our process space.

Source/Core/Common/MemArena.h

@@ -19,8 +19,8 @@
 class MemArena
 {
 public:
-	void GrabLowMemSpace(size_t size);
-	void ReleaseSpace();
+	void GrabSHMSegment(size_t size);
+	void ReleaseSHMSegment();
 	void *CreateView(s64 offset, size_t size, void *base = nullptr);
 	void ReleaseView(void *view, size_t size);