[mach-o] enhance arm64 reloc parser to support local pointer relocations

The arm64 assembler almost always uses r_extern=1 relocations in which the
r_symbolnum field is the index of the symbol the relocation references.  But
sometimes it will set r_extern=0 in which case the linker needs to read the
content of the reloction to determine the target.

Add test case that the r_extern=0 relocation round trips.

llvm-svn: 222198