message: Add option for ABI syscall function register in ISA info
diff: @@ -25,6 +25,7 @@ public:
  unsigned bytes() const { return bits() / CHAR_BIT; }
  virtual int spReg() const { return -1; } // Stack pointer
  virtual int gpReg() const { return -1; } // Global pointer
  + virtual int syscallReg() const { return -1; } // Syscall function register
  // GCC Compile command architecture and ABI specification strings
  virtual QString CCmarch() const = 0;
  @@ -135,6 +136,7 @@ public:
  unsigned int bits() const override { return 32; }
  int spReg() const override { return 2; }
  int gpReg() const override { return 3; }
  + int syscallReg() const override { return 17; }
  unsigned elfMachineId() const override { return EM_RISCV; }
  QString CCmarch() const override { return "rv32im"; }
  