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.

AK/StdLibExtras.cpp

@@ -5,6 +5,7 @@
 
 extern "C" {
 
+#ifndef KERNEL
 void* mmx_memcpy(void* dest, const void* src, size_t len)
 {
     ASSERT(len >= 1024);
@@ -51,6 +52,7 @@ void* mmx_memcpy(void* dest, const void* src, size_t len)
         memcpy(dest_ptr, src_ptr, len);
     return dest;
 }
+#endif
 
 #ifdef KERNEL
 

AK/StdLibExtras.h

@@ -11,14 +11,18 @@
 
 #include <AK/Types.h>
 
+#ifndef KERNEL
 extern "C" void* mmx_memcpy(void* to, const void* from, size_t);
+#endif
 
 [[gnu::always_inline]] inline void fast_dword_copy(dword* dest, const dword* src, size_t count)
 {
+#ifndef KERNEL
     if (count >= 256) {
         mmx_memcpy(dest, src, count * sizeof(count));
         return;
     }
+#endif
     asm volatile(
         "rep movsl\n"
         : "=S"(src), "=D"(dest), "=c"(count)

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;