start moving `zig cc` to stage2

 * build.zig: repair the ability to link against llvm, clang, and lld
 * move the zig cc arg parsing logic to stage2
   - the preprocessor flag is still TODO
   - the clang arg iterator code is improved to use slices instead of
     raw pointers because it no longer has to deal with an extern
     struct.
 * clean up error printing with a `fatal` function and use log API
   for messages rather than std.debug.print
 * add support for more CLI options to stage2 & update usage text
   - hooking up most of these new options is TODO
 * clean up the way libc and libc++ are detected via command line
   options. target information is used to determine if any of the libc
   candidate names are chosen.
 * add native library directory detection
 * implement the ability to invoke clang from stage2
 * introduce a build_options.have_llvm so we can comptime branch
   on whether LLVM is linked in or not.

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