Fix a nomenclature error in llvm-nm.

What llvm-nm prints depends on the file format. On ELF for example, if the
file is relocatable, it prints offsets. If it is not, it prints addresses.
Since it doesn't really need to care what it is that it is printing, use the
generic term value.

Fix or implement getSymbolValue to keep llvm-nm working.
Workaround for collision between enum members in LLVM's MachO.h and system headers
on Mac OS X (in particular mach/machine.h).

<rdar://problem/16494607>