Only check _ZN function prefix in Linux and FreeBSD targets in SymbolFileDWARF

In http://reviews.llvm.org/D9754 I enabled the mangled symbol name lookup
workaround used to find global and anonymous namespace symbols in linux binaries
for all platforms, however we should still only check for these symbols when
processing Linux or FreeBSD binaries where they are relevant. This patch makes
this change.

Test Plan: The tests from the original revision still pass:
TestCallCPPFunction.py
TestCallStopAndContinue.py
TestExprs.py
TestExprsChar.py
TestNamespace.py
TestOverloadedFunctions.py
TestRvalueReferences.py
TestThreadExit.py

Differential Revision: http://reviews.llvm.org/D9782

llvm-svn: 237467