kernel: 5.15: x86-64: enable CONFIG_SLS

Starting with GCC 12, we have the possibility of mitigating straight-line
speculation vulnerabilities in x86-64 targets. Make it so.

Reported-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>

target/linux/x86/64/config-5.15

@@ -402,6 +402,7 @@ CONFIG_SENSORS_K8TEMP=y
 CONFIG_SENSORS_VIA_CPUTEMP=y
 CONFIG_SERIAL_8250_PNP=y
 CONFIG_SERIAL_MCTRL_GPIO=y
+CONFIG_SLS=y
 CONFIG_SMP=y
 CONFIG_SOCK_RX_QUEUE_MAPPING=y
 CONFIG_SPARSEMEM=y