Now I found mbed library source files and try to understand where does it hangs. But now I completely confused why my program even goes into main. I checked startup file, there is first called SystemInit and after this _start (main). So I looked into SystemInit and saw that inside it's trying to start external 32kHz crystal with this code:

    // Start the external 32khz crystal oscillator.
    NRF_CLOCK->LFCLKSRC             = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);
    NRF_CLOCK->EVENTS_LFCLKSTARTED  = 0;
    NRF_CLOCK->TASKS_LFCLKSTART     = 1;

    // Wait for the external oscillator to start up.
    while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {
        // Do nothing.
    }

But it should hang in while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) loop infinitely in my case when I try to start my board without external 32kHz crystal.
How can it execute my code in main if it shouldn't get there?
Though I tried to change clock source from CLOCK_LFCLKSRC_SRC_Xtal to CLOCK_LFCLKSRC_SRC_RC (internal RC) but it didn't help with hanging in wait() problem.

UPD:

No, it seems I just don't understand how mbed libraries works. I thought if I rebuild mbed-src library with my changes and replace object files then this changes will be applied.
But now I just deleted system_nrf51822.o from the project and it still compiles without errors.
I'm at a loss.

UPD:

No, it's ok, I just needed to made some changes it main to trigger recompile.

UPD:

Ok, now I figured out what's what.
1. Previously I changed GCC_ARM object files, but by default mbed using files generated by keil ARM_STM. Now I changed object files from original mbed libary to the files with my changes and it works.
2. Original binary mbed library differs from the mbed-src version.
Original binary library in SystemInit start LFCLK source from internal RC or synthesized source so it doesn't hangs there, but in mbed-src version there is external LFCLK crystal trying to start.
3. Now I changed in SystemInit to start from internal RC LFCLK source, but it still hangs in the wait() function in the main as before.
So the question about wait() hang is still pending.

UPD:

Now situation is like this:
Last mbed library build from here works fine on my board with external 32kHz crystal.
Last mbed library source from here without any changes on my board with external 32kHz crystal hangs on wait() with this test code:

#include "mbed.h"

#define LED0_PIN_NO  p18
#define LED1_PIN_NO  p19

DigitalOut led0(LED0_PIN_NO);
DigitalOut led1(LED1_PIN_NO);

int main(void)
{
    led0 = 0;
    led1 = 0;
    
    led0 = 1;
    wait(1);
    led1 = 1;
    while(1);   
}

UPD:

Ok, last official build revision sources still works fine from here for board with external LFCLK crystal, but hangs without it.

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