Use consistent style for wasi-libc C source files. (#131)

Use consistent style for wasi-libc C source files. (#131)

For now, this means using `//`-style comments in .c source files (though
not public header files), and spaces rather than tabs. No strong opinion
here; this is just what the majority of the current code is using.

This also synchronizes basics/crt/crt1.c with libc-bottom-half's
version, though this is just a cleanup as the former isn't currently used
by the main wasi-libc build.