[RISCV] Fix the problem of parsing long version numbers

[sunshaoce]

Summary

For example, when parsing Zbpbo0p911, an error will be reported:
"multi-character extensions must be separated by underscores"

[asb]

LGTM, thanks.

llvm/lib/Support/RISCVISAInfo.cpp

static Error getExtensionVersion(StringRef Ext, StringRef In, unsigned &Major,
   Major = 0;
   Minor = 0;
   ConsumeLength = 0;
   MajorStr = In.take_while(isDigit);
   In = In.substr(MajorStr.size());
 
   if (!MajorStr.empty() && In.consume_front("p")) {
     MinorStr = In.take_while(isDigit);
-    In = In.substr(MajorStr.size() + 1);
+    In = In.substr(MajorStr.size() + MinorStr.size() - 1);
 
     // Expected 'p' to be followed by minor version number.
     if (MinorStr.empty()) {
       return createStringError(
           errc::invalid_argument,
           "minor version number missing after 'p' for extension '" + Ext + "'");
     }
   }