The GPL v3 text says
in reply to Re^4: Am I Allowed to Make a New Compiler/Language using Perl?
in thread Am I Allowed to Make a New Compiler/Language using Perl?
The output from running a covered work is covered by this License only if the output, given its content, constitutes a covered work.
and I have no idea what that means.
There's an interesting FAQ entry, which contains the sentence
If the user uses your program to enter or convert his own data, the copyright on the output belongs to him, not you
Which seems to imply that if you write a compiler and license it under the GPLv3, the output still belongs to the user. Comforting.