ch32v: board JSON march/mabi are dead — every series compiles as RV32EC/ilp32e, IMAC parts get software multiply

[zackees]

Part of the CH32V pipeline audit (#1102).

What happens

Every CH32V board JSON carries the ISA/ABI for its core, e.g. genericCH32V203C8T6.json:

"march": "rv32imacxw",
"mabi": "ilp32",

but nothing reads these fields. BoardConfig (fbuild-config) has no march/mabi members (grep: zero hits outside the JSON assets), and get_ch32v_config_for_mcu() in crates/fbuild-build-mcu/src/ch32v/mcu_config.rs returns the CH32V003 base config for every series:

// All CH32V variants currently share the CH32V003 config as a base,
// with march/mabi overridden from the board JSON extra_flags.   <-- this override does not exist
let json = match mcu {
    "ch32v003" => CH32V003_JSON,
    _ => CH32V003_JSON,
};

configs/ch32v003.json pins -march=rv32ec_zicsr -mabi=ilp32e, and extra_flags only ever contributes -D defines (BoardConfig::get_defines). Net effect: CH32V103, V203, V208, V303, V307, X035 and L103 — all RV32IMAC-class QingKe V3/V4 cores per WCH's datasheets — are compiled and linked as RV32EC with the ilp32e ABI.

Why it matters — worked example

int32_t c = a * b; on a CH32V203 (144 MHz, hardware multiplier) compiles to a call into libgcc's software __mulsi3 instead of a single mul instruction — a ~20x slowdown on multiply-heavy code (filters, LED color math). Division is worse. On top of that only registers x0–x15 are ever allocated, and the C++ -isystem multilib path (derived in orchestrator.rs from the same base config) selects the rv32ec/ilp32e headers, so the whole toolchain is consistently building for the wrong ISA. Builds pass and binaries run (RV32EC is a subset of RV32IMAC), which is why CI never noticed.

Notes for the fix

• Parse build.march/build.mabi into BoardConfig and inject them (overriding the base config's pair) into compile + link + the multilib get_cxx_system_includes() call.
• The JSON value rv32imacxw is a MounRiver-ism: the xw (WCH compressed) extension is not accepted by the xPack riscv-none-elf-gcc fbuild installs — strip xw (use rv32imac, or rv32imac_zicsr per toolchain version) or translate when building the flag, otherwise the wire-up will break every non-003 board at configure time.
• get_ch32v_config_for_mcu(mcu: &str) is actually called with the series string — rename the parameter while touching this.

Severity: HIGH (large silent performance/codegen regression on 7 of 8 supported series; misleading comment claims the override exists).