I found the official library here: http://www.atmel.com/tools/atmel-arm-toolchain.aspx
UPDATE:
I have finally understood understand how exactly Arduino accesses specific ARM features. It includes sam.h file which in turn contains Cortex Microcontroller Software Interface Standard. With this cool interface anyone can use so many cool features:
- Timers
- Advanced interrupts
- Power saving mode
- SIMD instructions
- And many other