Define OSPLAT for riscv64 and riscv32

[kxxt]

Proposed changes

Define OSPLAT for riscv64 and riscv32. Fixes boostorg/context#243 #340

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[olk]

On older compiler __riscv__ is defined - maybe this should be checked too.

[kxxt]

On older compiler __riscv__ is defined - maybe this should be checked too.

Thanks! I have updated it.