[RISCV] Support Shadow Call Stack

Currenlty assume x18 is used as pointer to shadow call stack. User shall pass
flags:

"-fsanitize=shadow-call-stack -ffixed-x18"

Runtime supported is needed to setup x18.

If SCS is desired, all parts of the program should be built with -ffixed-x18 to
maintain inter-operatability.

There's no particuluar reason that we must use x18 as SCS pointer. Any register
may be used, as long as it does not have designated purpose already, like RA or
passing call arguments.

Differential Revision: https://reviews.llvm.org/D84414