I could successfully build my own compiler with the help of https://github.com/esp8266/esp8266-wiki/wiki/Toolchain#install-the-xtensa-crosstool-ng-as-local-user and http://wiki.linux-xtensa.org/index.php/Crosstool-NG. Unfortunately I hit some problems in the compiler and I have to switch to gcc-5.3. Is there anybody porting the xtensa toolchain to gcc-5.3?
Is there anybody else interested in programming the ESP8266 in Ada? I'd be glad if I could integrate the porting result into the general toolchain.