target-i386: key sfence availability on CPUID_SSE, not CPUID_SSE2

sfence was introduced before lfence and mfence.  This fixes Linux
2.4's measurement of checksumming speeds for the pIII_sse
algorithm:

md: linear personality registered as nr 1
md: raid0 personality registered as nr 2
md: raid1 personality registered as nr 3
md: raid5 personality registered as nr 4
raid5: measuring checksumming speed
   8regs     :   384.400 MB/sec
   32regs    :   259.200 MB/sec
invalid operand: 0000
CPU:    0
EIP:    0010:[<c0240b2a>]    Not tainted
EFLAGS: 00000246
eax: c15d8000   ebx: 00000000   ecx: 00000000   edx: c15d5000
esi: 8005003b   edi: 00000004   ebp: 00000000   esp: c15bdf50
ds: 0018   es: 0018   ss: 0018
Process swapper (pid: 1, stackpage=c15bd000)
Stack: 00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000
       00000000 00000000 00000000 00000000 00000000 00000000 00000000
00000000
       00000000 00000206 c0241c6c 00001000 c15d4000 c15d7000 c15d4000
c15d4000
Call Trace:    [<c0241c6c>] [<c0105000>] [<c0241db4>] [<c010503b>]
[<c0105000>]
  [<c0107416>] [<c0105030>]

Code: 0f ae f8 0f 10 04 24 0f 10 4c 24 10 0f 10 54 24 20 0f 10 5c
 <0>Kernel panic: Attempted to kill init!

Reported-by: Stefan Weil <sw@weilnetz.de>
Fixes: 121f3157887f92268a3d6169e2d4601f9292020b
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
