Make calculate new vl configurable

[rez5427]

I don't know whether sail-riscv needs this.
The spike uses VLMAX to calculate new vl, and sail-riscv uses (AVL + 1) / 2.

The spec says:

The number of bits implemented in vl depends on the implementation’s maximum vector
length of the smallest supported type. The smallest vector implementation with VLEN=32
and supporting SEW=8 would need at least six bits in vl to hold the values 0-32
(VLEN=32, with LMUL=8 and SEW=8, yields VLMAX=32).

[github-actions]

Test Results

396 tests  ±0   396 ✅ ±0   0s ⏱️ ±0s
  4 suites ±0     0 💤 ±0 
  1 files   ±0     0 ❌ ±0 

Results for commit 424c23d. ± Comparison against base commit 9d86f01.