aarch64: Avoid alloca in target attribute parsing

aarch64: Avoid alloca in target attribute parsing

The handling of the target attribute used alloca to allocate
a copy of unverified user input, which could exhaust the stack
if the input is too long.  This patch converts it to auto_vecs
instead.

I wondered about converting it to use std::string, which we
already use elsewhere, but that would be more invasive and
controversial.

gcc/
	* config/aarch64/aarch64.cc (aarch64_process_one_target_attr)
	(aarch64_process_target_attr): Avoid alloca.