nolibc header update for 5.2-rc1 (RISCV support)

This single commit adds support for the RISCV architecture to the
nolibc header file. Currently the file is only used by rcutorture but
Pranith Kumar who contributed it would like to have this work merged.

I only did some trivial tests to verify that it does not break x86,
which I consider sufficient since all the code is cleanly enclosed
inside a single #if/endif block.
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