fix a bug when parsing zvl part of isa string

[ddwolf]

strlen of "zvl" is 3, not 4

test

build spike and enable vector extension

../configure --prefix=$RISCV --with-isa=RV64IMAFDCV_zicntr_zihpm_zvl512
make -j 

write a test case

// filename: a.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main() {
  int vlenb = 0;
  asm volatile ("csrr %0, vlenb\n\t" : "=r"(vlenb));
  printf("vlenb is %d\n", vlenb);
}

compile and build the case

riscv64-unknown-elf-gcc a.c -march=rv64gcv -g -O0 -std=gnu99 -I$RISCV/sysroot/usr/include

run the case

spike  pk a.out -p1 --isa=rv64imafdchv_zicntr_zihpm_zvl64  --priv=msu --hartids=0 

result

vlenb is 64

[aswaterman]

The correct name of the extension is "zvl512b", not "zvl512", so the code does the right thing when the input is correct. But we are obviously doing a bad job of validating inputs here. I'll submit an alternate PR to fix it.