turn ACT led on

arm.c

#define GPIO_BASE       0x20200000UL

#define LED_GPFSEL      4
#define LED_GPFBIT      21
#define LED_GPSET       8
#define LED_GPCLR       11
#define LED_GPIO_BIT    15

volatile unsigned int* gpio;

int main(void)
{
    gpio = (unsigned int*)GPIO_BASE;

    while(1)
    {
        *(gpio+LED_GPSET) = (1 << LED_GPIO_BIT);
    }
}

arm.rs

#![feature(no_std, lang_items, start)]
#![crate_type = "staticlib"]
#![no_std]

static GPIO_BASE: usize = 0x20200000;
static LED_GPFSEL: usize = 4;
static LED_GPFBIT: usize = 21;
static LED_GPSET: usize = 8;
static LED_GPCLR: usize  = 11;
static LED_GPIO_BIT: usize = 15;

#[allow(dead_code)]
#[no_mangle]
pub fn main() -> ! {
    loop {
        unsafe {
            *((GPIO_BASE + LED_GPSET) as *mut u16) = 1 << LED_GPIO_BIT;
        };
    }
}

#[lang = "rust_begin_unwind"] extern fn rust_begin_unwind() {}
#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] extern fn panic_fmt() -> ! { loop {} }
#[no_mangle]
pub unsafe fn __aeabi_unwind_cpp_pr0() -> () {
    loop {}
}