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
Fix logic of pthread type sizes to match C headers.
Merge pull request #513 from ibuclaw/x32arch

Fix logic of pthread type sizes to match C headers.
Merge pull request #510 from dawgfoto/refactorAA

Refactor AA implementation
Change memcpy, memmove, and memset to pure
Also change some strxxx functions to pure
Merge pull request #515 from 9rnsr/fix_pure

Change memcpy, memmove, and memset to pure
opaque AA Range interface