PPC64: Load TOC register in fiber_switchContext.

The TOC pointer was saved but not loaded. The TOC pointer is nonvolatile
for local calls (inside the same module).
Fix dynamic thread vector (DTV) pointers on some platforms.

On PowerPC and Mips platforms the dynamic thread vector (DTV) pointers
point 0x8000 past the start of each TLS block. This commit adds support
for this offset.