fix bug introduced by 7cef6c46552a988c1c710a0a2404316a27ef1d43
stack_start used to be stack_start + stack_size, so re-setting stk to stack_start would set the pointer to the *end* of the stack instead of the beginning.
Merge pull request #30 from benpicco/15d6070ad639b695b0abeef7474bc59a316017ac

set proper start of the stack in thread_stack_init for arm_common