[RISCV] Make SCS prologue interrupt safe on RISC-V

Prior to this patch the SCS prologue used the following instruction
sequence.

```
s[w|d]  ra, 0(gp)
addi    gp, gp, [4|8]
```

The problem with this sequence is that an interrupt occurring between the
store and the increment could clobber the value just written to the SCS.

https://reviews.llvm.org/D84414#inline-813203 pointed out a similar
issues that could have affected the epilogue.

This patch changes the instruction sequence in the prologue to:

```
addi    gp, gp, [4|8]
s[w|d]  ra, -[4|8](gp)
```

The downside to this is that there is now a data dependency between the
add and the store.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D149099