Disable auto-vectorization.

Until https://gitlab.com/qemu-project/qemu/-/issues/1976 is fixed, this
is just causing too much confusion and wasted time.

Removing V from the architecture string has the same effect, but
requires some code changes (to be able to compile code that's
_explicitly_ using V), and makes it less obvious that this is just a
temporary workaround for a qemu bug.

Bug: http://b/320416684
Test: objdump
Change-Id: Iec7d63ab227b31f82f530dabbdc4f3aa54277f04