Disable V extension when compiler doesn't support it. (#317)

This allows the vast majority of these tests to work with compilers that
don't support the V extension yet, which is helpful for people who
aren't using a vector branch of the compiler.

Specifically, this will hopefully allow us to run regression tests
against OpenOCD on every change, per
https://github.com/riscv/riscv-openocd/pull/563.