libphobos: Fix build fails for powerpc-linux

libphobos: Fix build fails for powerpc-linux

As register names are required for darwin, but not accepted by gas
unless you use `-mregnames', they have been conditionally removed on
non-darwin targets.

To avoid duplicating large blocks of almost identical code, the inline
assembly is now statically generated.

libphobos/ChangeLog:

	* libdruntime/core/thread/osthread.d (callWithStackShell): Statically
	generate PPC and PPC64 asm implementations, and conditionally remove
	PPC register names on non-Darwin targets.