I found where the adapted toolchain compiler is :

~/Documents/buildroot/output/host/bin/arm-buildroot-linux-uclibcgnueabihf-gcc 

The command line is then very simple:

~/Documents/buildroot/output/host/bin/arm-buildroot-linux-uclibcgnueabihf-gcc -o hello hello.c

You can obviously add it to PATH environment variable.