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

llvm-svn: 260851