Allow extern variables to volatile

[ikskuh]

One use case for external volatile variables is mapping them to their hardware address via the linker script instead of using pointers. This makes code less cluttered:

const LED0 = 3;
extern volatile var PORTB: u8;

fn turnOnLed() void {
    PORTB |= (1<<LED0);
}

PORTB is defined in the linker script to the correct hardware address

Note that this use case needs to be considered with #3206.

Current state would be this:

const LED0 = 3;
const PORTB = @intToPtr(*volatile u8, 0x????);

fn turnOnLed() void {
    PORTB.* |= (1<<LED0);
}

Another proposal that might be relevant is #4284 which might still be required to use

[andrewrk]

Here's how to do this with status quo zig:

const LED0 = 3;
extern var PORTB_linker: u8; // relocate this in the linker script
const PORTB: *volatile u8 = &PORTB_linker;

fn turnOnLed() void {
    PORTB.* |= (1<<LED0);
}