Fix endian issue in core.demangle

Variable `num` of type `size_t` is viewed as an array of chars.
This does not work on a big endian architecture.
Add external implemented callWithStackShell for Linux/PPC64.

This is required for the garbage collector.

Conflicts:
	src/core/thread.d
Add Linux/PPC(64) to stat.d

The definition follows the Linux kernel. The change is required in
order to compile druntime on Linux/PPC(64).

Conflicts:
	src/core/sys/posix/sys/stat.d