Common: Move FPU-related helpers into Common namespace

Makes these utilities' namespace consistent with the majority of the
Common library.

Source/Core/Common/x64FPURoundMode.cpp

@@ -9,7 +9,7 @@
 #include "Common/CommonTypes.h"
 #include "Common/Intrinsics.h"
 
-namespace FPURoundMode
+namespace Common::FPU
 {
 // Get the default SSE states here.
 static u32 saved_sse_state = _mm_getcsr();
@@ -49,4 +49,4 @@ void LoadDefaultSIMDState()
 {
   _mm_setcsr(default_sse_state);
 }
-}  // namespace FPURoundMode
+}  // namespace Common::FPU