* 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.