[RuntimeDyld][PowerPC] Use global entry points for calls between sections.

Functions in different objects may use different TOCs, so calls between such
functions should use the global entry point of the callee which updates the
TOC pointer.

This should fix a bug that the Numba developers encountered (see
https://github.com/numba/numba/issues/2451).

Patch by Olexa Bilaniuk. Thanks Olexa!

No RuntimeDyld checker test case yet as I am not familiar enough with how
RuntimeDyldELF fixes up call-sites, but I do not want to hold up landing
this. I will continue to work on it and see if I can rope some powerpc
experts in.

llvm-svn: 329335