build: add RISC-V native case

[yskelg]

#7498 Add a case for RISC-V native builds.
Future RISC-V JIT development will be utilize added this case.

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CT Test Results

       5 files     155 suites   49m 22s ⏱️
1 628 tests 1 578 ✔️ 50 💤 0 ❌
2 184 runs  2 114 ✔️ 70 💤 0 ❌

Results for commit 08bfc42.

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[mikpe]

This will never match, since the case values are on the form "<arch>-<4-or-8>". Also it's not clear to me that RISC-V needs this kludge. Are there RISC-V systems that report themselves as riscv64 but generate 32-bit code by default, or report themselves as riscv32 but generate 64-bit code by default? If not, then you don't need to touch this bit.

The origin for was operating systems like Solaris that ran on 64-bit hardware but defaulted (at the time at least) to generating 32-bit code.

[yskelg]

Thanks for the code review @mikpe, I'll update the patch to follow your code review! :)