Use more conservative and realistic default for vstart_alu

Since this feature is optional and rather niche, don't enable it by
default.  This will aid in software development, since it will force
software developed on Spike to be compatible with a much greater set
of vector processors.

riscv/processor.cc

@@ -127,7 +127,7 @@ void processor_t::parse_varch_string(const char* s)
   size_t len = str.length();
   int vlen = 0;
   int elen = 0;
-  int vstart_alu = 1;
+  int vstart_alu = 0;
 
   while (pos < len) {
     std::string attr = get_string_token(str, ':', pos);