bsd.sys.mk: Silence a few GCC warnings for C++

[jhb]

Summary

These warnings were already marked with -Wno-error=, but they trigger
so many false positives in libc++ headers to be utterly useless for
C++ code, so disable them entirely for C++.

[dim]

Indeed, the libc++ headers are extremely noisy with g++. Upstream does not care about compiling with -Wsystem-headers, so it's extremely unlikely these warnings will ever be fixed.