As reported in https://llvm.org/bugs/show_bug.cgi?id=25496, on FreeBSD,

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 91177308-0d34-0410-b5e6-96231b3b80d8