An answer from here: https://solarianprogrammer.com/2016/09/22/compiling-gcc-6-macos/
You can safely ignore all these warnings, these are related to the assembly code generated by GCC and have nothing to do with your C++ code.
Unfortunately, at this time, there is no way in which to instruct GCC not to use __textcoal_nt, or silence the above warnings. A quick and dirty workaround is to filter the output of the compiler with something like:
g++-6 main.cpp -o main 2>&1 >/dev/null | grep -v -e '^/var/folders/*' -e '^[[:space:]]*\.section' -e '^[[:space:]]*\^[[:space:]]*~*'
