Which is cause by:
$ eu-readelf --debug-dump=info a.out | less
...
Compilation unit at offset 144:
Version: 4, Abbreviation section offset: 91, Address size: 8, Offset size: 4
[ 9b] compile_unit abbrev: 1
producer (strp) "GNU C17 9.1.1 20190611 [gcc-9-branch
revision 272147] -mtune=generic -march=x86-64 -g -flto=1234567890"
language (data1) C99 (12)
...
So it's encoded in debug info in producer ;) Let me investigate that..