[riscv] refactor functions that register read/write via progbuf

The motivation for this refactor is to fixup error handling for some
corner cases. These functions attempt to cache S0 register and only then
perform a bunch of extra checks to figure out if the requested register
is valid one in this context. The problem is that there are few corner
cases when _*progbuf functions could receive a GPR as an input. For
example, an abstract read could fail (for whatever reason) leading to
infinite recursion:

````
save S0 -> read S0 -> save S0 -> read S0 -> ...
```

The case described above could be fixed by adding extra sanitity checks,
however I decided to make these functions more modular since I find
self-contained functions easier to read.

Change-Id: I01f57bf474ca45ebb67a30cd4d8fdef21f307c7d
Signed-off-by: Parshintsev Anatoly <anatoly.parshintsev@syntacore.com>