Only track C++ stdlibs for C++ code

After a formula is built, scan all mach-o files for dynamic links
to see if any of them point to a C++ stdlib (libc++ or libstdc++).
If one of them is linked, record that information in the formula's tab.

This replaces the old behaviour where all files were assumed to be C++
code, and stdlibs were always tracked regardless of whether they were
actually linked against.

This also modifies the way that tabs are written - now tabs are written
with the stdlib field null, and values are only written if an stdlib
is detected.
CxxStdlib: allow for nil stdlibs

A nil stdlib value represents non-C++ code.
Track the OS's default C++ stdlib