As reported in https://llvm.org/bugs/show_bug.cgi?id=25496, on FreeBSD,
C++ programs compiled for profiling (using `-pg`) should be linked with
`-lc++_p` (or `-lstdc++_p`, depending on the `-stdlib=` setting), not
with the regular C++ libraries.
Add a `FreeBSD::AddCXXStdlibLibArgs()` override to handle this, and add
a test case for it. While here, extend the test case for the proper
passing of -lm and -lm_p.
Reviewers: compnerd, davide, dws, emaste
Reviewed By: compnerd
Differential Revision: http://reviews.llvm.org/D16264
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260851
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