libpmc: Use LIB_CXX instead of explicit LDADD to link a C++ library.

This uses the C++ compiler as the linker instead of the C compiler
letting the compiler driver pick the right libraries. This is a no-op
on main and stable/13 but matters for stable/12 where the current
logic breaks for external GCC since it tries to use a non-existent
libstdc++.

Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39197