sim: fix multiple x extension issue

allow  --isa=rv32gc_xdummy_xabcd_xdef

Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>

riscv/processor.cc

@@ -244,7 +244,7 @@ void processor_t::parse_isa_string(const char* str)
         p++;
       } else if (*p == 'x') {
         const char* ext = p + 1, *end = ext;
-        while (islower(*end) || *end == '_')
+        while (islower(*end))
           end++;
 
         auto ext_str = std::string(ext, end - ext);