Implement fibers on Linux/PPC64.

As a first attempt I am using the Posix version with ucontext_t.
Partial fix to compile on X86
Final fix for X86
Merge pull request #506 from redstar/ppc_thread

Implement fibers on Linux/PPC64.
Change the Exception/Error constructors to @safe pure nothrow

This is a druntime fix-up for the compiler bug 10254.
Merge pull request #512 from 9rnsr/fix10254

Change the Exception/Error constructors to @safe pure nothrow