fix(toolchain): resolve RISC-V C++ multilib from GCC

[zackees]

Closes #1118\n\nUse GCC's -print-multi-directory result when constructing the xPack C++ multilib include path. This handles the default . multilib used by rv32imac/ilp32 and preserves extension-bearing directories such as
v32imafc_zicsr/ilp32f.\n\nValidation: soldr cargo test -p fbuild-toolchain toolchain::riscv --lib (9 passed); soldr rustfmt --check crates/fbuild-toolchain/src/toolchain/riscv.rs.

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• crates/fbuild-toolchain/src/toolchain/riscv.rs

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