sw:vec_iree: Use explicit zvl and zve ISA set to generate vector ops

Explicitly set zvl512b to support "--riscv-v-vector-bits-min=512",
and zve32x to support "--riscv-v-fixed-length-vector-elen-max=32" for
integer {8, 16, 32} types. By using these ISA set instead of the
top-level "v", we can ensure the compiler doesn't generate "vfxx" ops,
even when processing float models.

See https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#sec-vector-extensions

Change-Id: I5370bc17b70fc10514145fa1a42688ef5cfffbbf