[Flang][RISCV] Set vscale_range based off zvl*b (#77277)

This patch implements the logic (for now, copied from
RISCVTargetInfo::getVScaleRange) so that we can compute the vscale_range
based off of the zvl*b extension, e.g. using an arch with zvl256b now
implies vscale_range(2,1024).

It's worth noting that we don't have to exactly copy the behaviour of
clang with regards to how it interacts with the
-mvscale-min/-mvscale-max flags, but changing it can be left to a future
patch.

This also adds a guard for +sve so that we only check for it on aarch64,
which was the behaviour prior to 898db113