gcc spectre stuff for netbsd-8

[maya%netbsd.org@localhost]

Hi,

I've done the following, hopefully it will enable others.

Completed:
- Builds amd64 tools
- Builds working amd64 kernel (*without new mitigations*!)

Need:
- Test full release, stress test
- Test i386 too
- Decide which spectre mitigation to use
- Test that kernel with that mitigation works
- Also, modules!
- Run the new testcases from GCC testsuite which were added with this
  feature

Maybe:
- Figure out what I just did to the documentation?

Patch: http://netbsd.org/~maya/netbsd8-gcc-spectre-20180312


(Originally from opensuse's gcc patch, and comparing a lot to gcc-HEAD
where it was insufficient)