According to the architecture reference manual, 14.7.3 Stack Alignment:

``Stacks may have arbitrary byte alignment, but performance may suffer
if at least octaword alignment is not maintained by software.''

(where octaword is 128 bits)

So while the existing value is not wrong, raising it, rather than
changing gcc defaults, is a better way to make sure code which assumes
128-bit alignment will keep running.