Build stage-1 gcc with ubsan

[Prathamesh Kulkarni]

Hi,
For bootstrap gcc build with ubsan, we have --with-build-config=bootstrap-ubsan,
however I was wondering how to build only stage-1 gcc with ubsan ?
I tried:
CFLAGS="-fsanitize=undefined" CXXFLAGS="-fsanitize=undefined" make -j8
but that doesn't seem to work.

Thanks,
Prathamesh