fix(toolchain): riscv g++ -isystem set misses the default multilib dir - rv32imac CH32V C++ builds fail on Windows

[zackees]

Severity: CRITICAL — every rv32imac CH32V C++ build (V103/V203/V208/V303/V307/X035/L103) fails on Windows. The bench-prep box is Windows.

Problem

get_cxx_system_includes in crates/fbuild-toolchain/src/toolchain/riscv.rs:77-139 builds the C++ -isystem list (the workaround for the known Windows xPack g++ header-resolution breakage) by pushing <ver>/riscv-none-elf/<march>/<mabi>. But xPack riscv-none-elf-gcc 14.2.0-3 has no rv32imac/ilp32 multilib subdirectory — that combination is the toolchain's default multilib (-print-multi-directory → .), whose bits/c++config.h lives at <ver>/riscv-none-elf/ directly. The nonexistent path is silently skipped, so the compile fails.

Until PR #1113, this was masked: the dead-march bug compiled everything as rv32ec_zicsr/ilp32e, and the rv32ec/ilp32e multilib dir does exist. #1113 fixed the ISA selection and thereby exposed this on all rv32imac boards.

Evidence (verified empirically on this machine)

• riscv-none-elf-g++ -march=rv32imac_zicsr -mabi=ilp32 <file.cpp> with fbuild's exact -isystem list → bits/c++config.h: No such file or directory.
• Adding -isystem <ver>/riscv-none-elf → compiles.
• riscv-none-elf-gcc -march=rv32imac_zicsr -mabi=ilp32 -print-multi-directory → . (default multilib).

Why CI missed it

All build-ch32*.yml run via template_build.yml on ubuntu-latest only, where GCC resolves its own headers and the -isystem workaround is unnecessary. There is no Windows platform-build lane for CH32V.

Secondary defect (same function)

The multilib-dir heuristic march.split('_').next() is wrong in general: real F-extension multilib dirs keep _zicsr (e.g. rv32imafc_zicsr/ilp32f), so a future hard-float board (see the V30x FPU issue) hits the same silent skip.

Fix direction

Resolve the multilib dir by invoking gcc -march=<m> -mabi=<a> -print-multi-directory and map . → <ver>/riscv-none-elf; or minimally, fall back to pushing <ver>/riscv-none-elf whenever the computed multilib dir does not exist. Prefer the -print-multi-directory approach — it kills the heuristic too.

Acceptance

• On Windows: soldr cargo run -p fbuild-cli -- build tests/platform/ch32v203 -e ch32v203 succeeds (C++ TUs compile).
• tests/platform/ch32v003 still builds (rv32ec path unchanged).
• Consider adding a windows-latest lane to at least one CH32V build workflow so this class of failure is CI-visible.

Found during the 2026-07-21 CH32V bench-readiness sweep.

[zackees]

Parent meta: #1130 (2026-07-21 CH32V bench-readiness sweep).