libc: define max_align_t

Needed by newer libstdc++.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
runtime: define __stack_chk_guard

Needed by newer libstdc++.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Switch to newer gcc

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>