Simplify multitasking

[bugadani]

My hypothesis is that we can get rid of the FIRST_SWITCH flag, saving a few instructions on each task switch.

I hope this works on RISC-V but we can restore the flag on it if not.

[bjoernQ]

Works fine on all targets - just one thing:

warning: unused import: `core::sync::atomic::Ordering`
 --> esp-wifi\src\timer\riscv.rs:2:5
  |
2 | use core::sync::atomic::Ordering;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

[bjoernQ]

LGTM!