fix Issue 7954 - x86_64 Windows fibers do not save nonvolatile XMM registers
Win64 fibers: changed layout of context stack to allow aligned SSE2 instructions

Implemented a trampoline function to prevent differently-aligned stacks on first call to fiber entry point.
Win64 Fibers: implement changes discussed in PR 810
Merge pull request #810 from jblume/fibers-xmm

fix Issue 7954 - x86_64 Windows fibers do not save nonvolatile XMM registers