Move large stack buffer in bssl_shim to heap.

The size of the stack caused by this object is problematic for systems
that have smaller stacks because they expect many threads.

Change-Id: Ib8f03741f9dd96bf474126f001947f879e50a781
Reviewed-on: https://boringssl-review.googlesource.com/5831
Reviewed-by: Matt Braithwaite <mab@google.com>
Reviewed-by: Adam Langley <agl@google.com>