Instead of checking for absolute paths and current working directories
in various file system operations, there is one simple solution: allow
overriding `std.fs.cwd` on WASI.

os.realpath is back to causing a compile error when used on WASI. This
caused a compile error in the Sema handling of `@src()`. The compiler
should never call realpath, so the commit that made this change is
reverted (95ab942184427e7c9b840d71f4d093931e3e48fb). If this breaks
debug info, a different strategy is needed to solve it other than using
realpath.

I also removed the preopens code and replaced it with something much
simpler. There is no longer any global state in the standard library.

Additionally-
 * os.openat no longer does an unnecessary fstat on WASI when O.WRONLY
   is not provided.
 * os.chdir is back to causing a compile error on WASI.