[RISCV] Adding vlenb register as callee register (llvm#165796)

In recent debug sessions we noticed that GDB debugger is showing more
stack trace than lldb.
After enabling unwinding log, it seems like the issue is that the CFA is
dependent on the value of vlenb.
vlenb doesn't change in runtime so we can assume its value from frame 0.

Co-authored-by: Bar Soloveychik <[email protected]>

Author: barsolo2000

lldb/source/Plugins/ABI/RISCV/ABISysV_riscv.cpp

@@ -798,6 +798,8 @@ bool ABISysV_riscv::RegisterIsCalleeSaved(const RegisterInfo *reg_info) {
           .Cases({"f8", "f9", "f18", "f19", "f20", "f21", "f22", "f23"},
                  is_hw_fp)
           .Cases({"f24", "f25", "f26", "f27"}, is_hw_fp)
+          // vlenb is constant and needed for vector unwinding.
+          .Case("vlenb", true)
           .Default(false);
 
   return is_callee_saved;