Allow to properly override `get_linker_section` and `get_section_flags` in `asm` and `hasm` files

[AngheloAlf]

Those segments were hardcoding '.section .text, "ax"', which were making a bit more tedious to write custom sections that extended splat's asm section. Specifically I had to duplicate the get_file_header function https://github.com/AngheloAlf/goin_quackers/blob/8441ce1b5fdaa95fe3ac6f32f6a95acc68603008/tools/splat_ext/init.py#L14