e-lib: Eliminate potential barrier deadlock

[olajep]

Deadlock if core0 calls e_barrier_init() after some other core calls e_barrier()

Force user to have bar_array in .bss

Then we can eliminate this:
https://github.com/adapteva/epiphany-libs/blob/2015.1/src/e-lib/src/e_mutex_barrier_init.c#L38
which can overwrite flag set here:
https://github.com/adapteva/epiphany-libs/blob/2015.1/src/e-lib/src/e_mutex_barrier.c#L99
and then core0 gets stuck here:
https://github.com/adapteva/epiphany-libs/blob/2015.1/src/e-lib/src/e_mutex_barrier.c#L87

Depends on #54

Unlikely to happen on hw for "normal programs" (e.g., SIMD)