[Bug target/124140] gcc/doc/riscv-ext.texi does not have an explicite license

[doko at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124140

--- Comment #1 from Matthias Klose <doko at gcc dot gnu.org> ---
same for riscv-mcpu.texi and riscv-mtune.texi