Fixed invalid assumption that "char" type is signed.

Nothing in any C++ standard guarantees that "char" is signed (when compiling for ARM, for instance, it is typically unsigned).
Set the cluster_arch_bitsize to "32bit" on ARM.
Implemented coroutine context-switching for ARM.
ARM in configure
Add some cross-compiling support to the configure script
Add boost to fetch list
Improve cross-compiling support in configure script
Fix defects from review
Merge branch arm into next

Review 1228 by @danielmewes
Closes #1625
Closes #227