Fix exports in core::libc

[catamorphism]

core::libc has FIXMEs about cleaning up exports. I'm not sure I exactly understand what needs to be done, so am just filing it.

[graydon]

This is probably blocked on rewriting resolve. I want the exports from libc to have the following characteristics:

• There is a list of explicit exports-from-libc that is statically checked to ensure those definitions are present on all platforms. In the rewrite this will probably take the form of a list of pub import ... directives.
• There are a bunch of residual globs that pick up and re-export everything else beyond the intersection list, so that when you're on platform foo that has a libc definition funcs::os::extras::bar() you can just call libc::bar() and it'll build on that platform. But you will have made platform-specific code by doing so. This will be a list of pub import ...* globs.

Currently, export globs don't work terribly well. They kinda work for exporting a bunch of items, but not for re-exporting a glob-import from somewhere else. And I think there was an ordering problem of some sort. I eventually gave up and left it as-is, with the few platform-specific cases explicitly glob-importing definitions from the inner modules of libc. Which is undesirable in the longer term.

[catamorphism]

No longer blocked on resolve.

[graydon]

So minor! Yet blocked on pub use not quite working right.