[lldb] Update the check for Linux or FreeBSD in SymbolFileDWARF::FindFunctions

This diff
1. Adds a comment to ObjectFileELF.cpp about the current
approach to determining the OS.
2. Replaces the check in SymbolFileDWARF.cpp with a more robust one.

Test plan:
Built (on Linux) a test binary linked to a c++ shared library
which contains just an implementation of a function TestFunction,
the library (the binary itself) doesn't contain ELF notes 
and EI_OSABI is set to System V.
Checked in lldb that now "p TestFunction()" works fine 
(and doesn't work without this patch).

Differential revision: https://reviews.llvm.org/D27380

llvm-svn: 288687