cmake: add -fcheck-new to cxx_compile_flags
cmake: add -fcheck-new to cxx_compile_flags

GCC assumes that operator new never returns null, but throws an exception
instead. This is defined by the C++ standard, and thus ok. But we disable
exceptions with -fno-exceptions, so we break this assumption. GCC then goes
ahead and removes some of our nullptr checks. This flag removes the
assumption.

This adds ~1.4kB to the binary size of the px4fmu-v4 target.

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