See #5854
Some tiny tweaks too:
* Use `wasm-freestanding-musl` instead of `wasm32-freestanding-musl`,
making it pointer-size-agnostic.
* Fix trying to build non-existent wasm musl start files.
* std.log: still print error messages in ReleaseSmall builds.
- when start code gets an error code from main, it uses std.log.err
to report the error. this resulted in a test failure because
ReleaseSmall wasn't printing `error: TheErrorCode` when an error
was returned from main. But that seems like it should keep working.
So I changed the std.log defaults. I plan to follow this up with a
proposal to change the names of and reduce the quantity of the
log levels.
* warning emitted when using -femit-h when using stage1 backend; fatal
log message when using -femit-h with self-hosted backend (because the
feature is not yet available)
* fix double `test-cli` build steps in zig's build.zig
* update docgen to use new CLI
* translate-c uses `-x c` and generates a temporary basename with a
`.h` extension. Otherwise clang reports an error.
* --show-builtin implies -fno-emit-bin
* restore the compile error for using an extern "c" function without
putting -lc on the build line. we have to know about the libc
dependency up front.
* Fix ReleaseFast and ReleaseSmall getting swapped when passing the
value to the stage1 backend.
* correct the zig0 CLI usage text.
* update test harness code to the new CLI.
