unittests/3DNow: Fix femms test

We are onyl wanting to test the FTW, which is 16-bit but we were reading
a whole 32-bits. The upper 16-bits was ending up in the reserved
section, which is undefined as to what it contains. In FEX it'll contain
0 but on a Zen 4 CPU it contains 0xFFFF.

Only read the bits we care about.

unittests/ASM/3DNow/0E.asm

@@ -23,6 +23,7 @@ femms
 mov rdx, 0xe0000000
 o32 fstenv [rdx]
 
-mov eax, dword [rdx + 8] ; Offset 8 in the structure has FTW
+mov eax, 0
+mov ax, word [rdx + 8] ; Offset 8 in the structure has FTW
 
 hlt