Turn off gcc's tree-slp-vectorize option

Turn off gcc's tree-slp-vectorize option

sadf crashes in non regression test nr. 00610 with a segmentation fault:

$ sadf -c tests/data-10.3.1 >/dev/null
file_magic: OK
HZ: Using current value: 100
file_header: OK
file_activity: A_CPU A_PCSW A_IRQ A_SWAP A_PAGE A_IO A_MEMORY A_HUGE A_KTABLES A_QUEUE A_DISK A_NET_DEV A_NET_EDEV A_NET_NFS A_NET_NFSD A_NET_SOCK A_NET_IP A_NET_EIP A_NET_ICMP A_NET_EICMP A_NET_TCP A_NET_ETCP A_NET_UDP A_NET_SOCK6 A_NET_IP6 A_NET_EIP6 A_NET_ICMP6 A_NET_EICMP6 A_NET_UDP6 A_PWR_CPU A_PWR_FAN A_PWR_TEMP A_PWR_USB A_FS OK
Statistics:
HRHCHnuunu...uuuunununu...uu......uu...n.n.n.n.Segmentation fault (core dumped)

It turns out that the problem is due to gcc optimizations made with
option -O2. Unsetting tree-slp-vectorize flag fixes that problem.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>