[PPC64] Fix PR19893 - improve code generation for local function addresses

Rafael opened http://llvm.org/bugs/show_bug.cgi?id=19893 to track non-optimal
code generation for forming a function address that is local to the compile
unit.  The existing code was treating both local and non-local functions
identically.

This patch fixes the problem by properly identifying local functions and
generating the proper addis/addi code.  I also noticed that Rafael's earlier
changes to correct the surrounding code in PPCISelLowering.cpp were also
needed for fast instruction selection in PPCFastISel.cpp, so this patch
fixes that code as well.

The existing test/CodeGen/PowerPC/func-addr.ll is modified to test the new
code generation.  I've added a -O0 run line to test the fast-isel code as
well.

Tested on powerpc64[le]-unknown-linux-gnu with no regressions.