[docs] Prefer --gcc-install-dir= to deprecated GCC_INSTALL_PREFIX (#85458)

Setting GCC_INSTALL_PREFIX leads to a warning (#77537).

Link:
https://discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link:
https://discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833