[RISCV] Support machine constraint "S"

Similar to D46745, "S" represents an absolute symbolic operand, which
can be used to specify the access models, e.g.

  extern int var;
  void *addr_via_asm() {
    void *ret;
    asm("lui %0, %%hi(%1)\naddi %0,%0,%%lo(%1)" : "=r"(ret) : "S"(&var));
    return ret;
  }

'S' is documented in trunk GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101275

Reviewed By: luismarques

Differential Revision: https://reviews.llvm.org/D105254