gawk: refactor

- Don't build with libsigsegv by default.  The build apparently attempted
  to link against it, but it never retained the reference anyway...
- Side effect: stdenv bootstrapping needs no libsigsegv anymore.
- Run checks, but only in the interactive gawk by default on Linux,
  so that stdenv bootstrap isn't slowed down (by glibc locales, etc.).
- xz should be no longer needed in inputs, as we have it in stdenvs now.

The whole change was triggered by some used kernel versions still
breaking libsigsegv tests #28464.