`riscv-rt`: Prune unused symbols

[piegamesde]

I'm trying to optimize my binary size. For this, I override all default functions that I don't need with stubs. However, most of these symbols are still present in my binary. Is there a way to get rid of them?

As far as I can tell, using weak symbols could resolve this, but I don't know much about linking. (Incomplete) list of symbols that I'd like to get rid of:

• __wfi
• __read_mhartid
• __read_mcause
• default_mp_hook
• DefaultExceptionHandler
• DefaultInterruptHandler
• default_setup_interrupts
• default_start_trap