refactor(ch32v): apply board ISA/ABI as a pure config -> config function

[zackees]

Replaces the in-place apply_board_isa(&mut config, march, mabi) and apply_board_isa_for_toolchain(&mut config, ...) with side-effect-free with_board_isa(config, ...) -> config and with_board_isa_for_toolchain(config, ...) -> config. These are composed from two named rules: march_for_toolchain (normalize for xPack, keep WCH's vendor extensions otherwise) and with_flag_value (set every -march=/-mabi= compiler-common and linker flag). The orchestrator applies the result in one line:

let mcu_config = with_board_isa_for_toolchain(get_ch32v_config_for_mcu(&series)?, march, mabi, &prefix);

Behaviour is unchanged. The existing tests are ported to the new signatures.

Same theme as the #1605 review: fixups should be pure state -> state functions applied once.

Local checks: fmt, clippy -p fbuild-build-mcu --all-targets -D warnings, and test -p fbuild-build-mcu (77 passed).

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 795ca58 and ee8da2c.

📒 Files selected for processing (4)

• crates/fbuild-build-mcu/src/ch32v/ch32v_compiler.rs
• crates/fbuild-build-mcu/src/ch32v/mcu_config.rs
• crates/fbuild-build-mcu/src/ch32v/orchestrator.rs
• crates/fbuild-build-mcu/src/ch32v/orchestrator_tests.rs

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