[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/cpu.s

@@ -0,0 +1,70 @@
+global get_mxcsr: function hidden (get_mxcsr.end - get_mxcsr)
+get_mxcsr:
+    push 0
+    stmxcsr [rsp]
+    pop rax
+    ret
+.end:
+
+global set_mxcsr: function hidden (set_mxcsr.end - set_mxcsr)
+set_mxcsr:
+    push rdi
+    ldmxcsr [rsp]
+    pop rax
+    ret
+.end:
+
+global get_xcr0: function hidden (get_xcr0.end - get_xcr0)
+get_xcr0:
+    xor rcx, rcx ; there is no dana there is only xcr0
+    xgetbv
+    ret ; technically edx has the high 32 bits, but bits 10+ are reserved
+.end:
+
+global set_xcr0: function hidden (set_xcr0.end - set_xcr0)
+set_xcr0:
+    xor rcx, rcx ; there is no dana there is only xcr0
+    mov rax, rdi ; technically edx should be or'd into the high bits, but xcr0 bits 10+ are resereved
+    xsetbv
+    ret
+.end:
+
+global get_rsp: function hidden (get_rsp.end - get_rsp)
+get_rsp:
+	mov rax, rsp
+	ret
+.end:
+
+global get_rip: function hidden (get_rip.end - get_rip)
+get_rip:
+	pop rax ; do the same thing as 'ret', except with 'jmp'
+	jmp rax ; with the return address still in rax
+.end:
+
+global get_caller: function hidden (get_caller.end - get_caller)
+get_caller:
+	; No prelude - don't touch rsp or rbp
+	mov rax, [rbp+8]
+	ret
+.end:
+
+global get_grandcaller: function hidden (get_grandcaller.end - get_grandcaller)
+get_grandcaller:
+	; No prelude - don't touch rsp or rbp
+	mov rax, [rbp]
+	mov rax, [rax+8]
+	ret
+.end:
+
+global get_gsbase: function hidden (get_gsbase.end - get_gsbase)
+get_gsbase:
+	rdgsbase rax
+	ret
+.end:
+
+global _halt: function hidden (_halt.end - _halt)
+_halt:
+	hlt
+	jmp _halt
+.end:
+