* 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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