Proper unwind for currently executing fiber

Each yielded fiber has a preserved coro state stored in a corresponding
variable however an executing fiber has a volatile state placed in CPU
registers (stack pointer, instruction pointer and non-volatile registers)
and corresponding context-storing variable value is invalid.
For already yielded fiber we have to use a special asm-written handler to make
a temporary switch to the preserved state and capture executing context what
is not needed for executing fiber.
After the patch for the executing fiber NULL is passed to the backtrace
function as coro context and then backtrace function could decide should
it use special context-switching handler or might just use unw_getcontext
from the unwind library.