* build-exe does include the startup code that supplies _start for the
wasm32-freestanding target. Previously this did not occur because
of logic excluding "freestanding".
* build-lib for wasm32-freestanding target gets linked by LLD. To avoid
infinite recursion, compiler_rt and zig libc are built as objects
rather than libraries.
- no "lib" prefix and ".wasm" extension instead of ".a". Rather than
build-lib foo.zig producing "libfoo.a", now it produces "foo.wasm".
* go back to using `.o` extension for webassembly objects
* zig libc only provides _start symbol for wasm when linking libc.
81e960eb74