mark .init and .trap sections as executable

This is needed for lld, otherwise it will complain about section flag
mismatch:

    ld.lld: error: incompatible section flags for .text
    >>> target/riscv32imac-unknown-none/debug/deps/libriscv_rt-7850ee1a6233fbe9.rlib(riscv_rt-7850ee1a6233fbe9.4tmuw4s4crjeqbm5.rcgu.o):(.trap): 0x4
    >>> output section .text: 0x6