* some manual fixes to generated CPU features code. In the future it
would be nice to make the script do those automatically.
* add to various target OS switches. Some of the values I was unsure of
and added TODO panics, for example in the case of spirv CPU arch.
This is a partial revert of 105db13536.
As we learned from Void Linux packaging, these options are not actually
helpful since the distribution package manager may very well want to
cross-compile the packages that it is building.
So, let's not overcomplicate things. There are already the standard
options: -Dtarget, -Dcpu, and -Ddynamic-linker.
These options are generally provided when the project generates machine
code artifacts, however, there may be a project that does no such thing,
in which case it makes sense for these options to be missing. The Zig
Build System is a general-purpose build system, after all.
