Fix compiler errors in std.process and std.Build.Step.Compile

[apwadkar]

Fixes

_ = &std.process.changeCurDirC;
_ = &std.Build.Step.Compile.dependsOnSystemLibrary;

from #20505.

Looks like chdirC was renamed chdirZ but that wasn't reflected in the std.process alias. I'm not sure if this alias is even needed, I couldn't find any references to changeCurDir in std.

For dependsOnSystemLibrary, it looks like because of the way it finds dependency libraries, compile can't be a *const Compile. And since Module::link_libc returns a ?bool with null meaning "doesn't matter", I used best judgement and assigned it to false based on how is_linking_libc is being used.

[andrewrk]

Thanks!