fep(sig-riscv): add experimental support FEP

[shiptux]

Summary

Adds FEP-34 (Provisional): an experimental track of RISC-V architecture support for the FlagOS software stack (compile adaptation + dependency analysis only).

Deliverables land as merged PRs in the originating Flag* repositories — no standalone PDF reports:

• Build helpers / CI matrix entries that enable riscv64 builds, or a documented "not yet supported" status (README, ARCH_SUPPORT-style file, or CI skip).
• Dependency-trimming PRs in originating repositories for deps that are unavailable, broken, or unnecessary on riscv64.
• Capability boundary per module = whatever the originating repository's own documentation says after the experiment lands.

Review routing

sig-riscv is proposed as a Planned SIG in #35 (charter draft, no Chair yet). Per the bootstrap note in fep/README.md, this FEP is reviewed directly by the TSC until sig-riscv activates. #35 landing first is preferred but not a hard blocker for review.

What this PR does

• Adds fep/sig-riscv/34-experimental-support.md (Provisional, FEP-34).

Scope boundaries

• sig-riscv (this FEP): RISC-V architecture-level compile adaptation + dependency analysis. Initial build-matrix priority within Wave 1: FlagCX, FlagTree, FlagGems, FlagScale; remaining seven Wave 1 modules follow.
• sig-os (#19): distribution-level packaging, supported distribution matrix, publishing channels, per-distribution install/runtime validation. Build helpers and compile evidence from sig-riscv feed into sig-os's workflow.
• sig-operator / module-owning SIGs (e.g. #33 for SpacemiT): vendor-specific RISC-V backends — operator implementations, runtime backends. Consumed by sig-riscv, not duplicated.

Notes

• Marked Draft while content firms up.