[kernel] Set mxcsr and xcr0 in cpu_early_init

There are some SSE instructions (moveaps, moveups) in userland code that
QEMU software emulation seems to be fine with but generate `#UD` on KVM.
So let's finally get floating-point support working. This is the first
step, just setting the control regs to try to fix that error.

src/kernel/kernel.module

@@ -16,7 +16,7 @@ kernel = module("kernel",
         "clock.cpp",
         "cpprt.cpp",
         "cpu.cpp",
-        "debug.s",
+        "cpu.s",
         "device_manager.cpp",
         "frame_allocator.cpp",
         "gdt.cpp",