Surely you will not get a perfect 100kHz from 11.059 Mhz xtal.

However if you use Mode2 auto-reload, you only need to do one toggle in your interrupt service routine.

UNTESTED:-

void timer0() interrupt 1 // interrupt address is 0x000b 
{ 
    P0_1 = !P0_1;      // 5uS hi / 5uS low makes 10uS period.
}
void main(void)
{
    TH1 = 0xFD;
    SCON = 0x50;           // for serial communication
    TMOD = 0x20 | 0x02;    // uart Timer1 auto-reload as well.
    TH0 = -5;              // every 5 clocks assuming 12 clocker.
    TR0 = 1;               // start timer
    EA = 1;                // start IRQ
    while (1) ;
}

Compile this and look at the generated code. If your compiler puts a lot of rubbish into ISR, then just re-write in assembler.
No registers need to be saved, because you can toggle the port pin by itself.

You only have 5 machine cycles to play with anyway including the interrupt vectoring which is between 3 and 8 machine cycles. SDCC compiles the ISR into CPL P0_1 and RETI which is 3 machine cycles.

So my conclusion is that it will work with a 6 clocker 8051 if you never use MUL or DIV. The vectoring is quicker. However your main routine will be fairly lame (IRQ 6-9 machine cycles leaving 1-4 cycles for MAIN)

If you have an 8051 variant that can do PWM then it is possible to do without interrupts.