Kernel: Don't use MMX memcpy() in the kernel.

I just discovered the hard way that clobbering FPU/MMX/SSE registers in the
kernel makes things very confusing for userspace (and other kernel threads.)

Let's banish all of those things from the kernel to keep things simple.

Kernel/StdLib.cpp

@@ -8,9 +8,10 @@ extern "C" {
 
 void* memcpy(void* dest_ptr, const void* src_ptr, size_t n)
 {
-    if (n >= 1024) {
+#ifndef KERNEL
+    if (n >= 1024)
         return mmx_memcpy(dest_ptr, src_ptr, n);
-    }
+#endif
 
     size_t dest = (size_t)dest_ptr;
     size_t src = (size_t)src_ptr;