This replaces an earlier kludge which was at the wrong level
and caused "GOARCH=386 tinygo test os" to fail to compile on linux.
Stubbing just the one missing function, syscall.seek, lets
os tests compile on linux 386, and skipping tests of seek and Fstat
(which has a cryptic dependency on syscall.Seek via time)
lets os tests pass on linux 386.
The stub can be removed once tinygo implements go assembly and picks up the real definition.
This is File.Stat from https://github.com/tinygo-org/tinygo/pull/2371,
plus the windows bits,
plus a smoke test more or less from upstream,
all pulled together and rebased by dkegel-fastly.
Also fix a couple os tests that wrote to current directory to write to os.TempDir() instead.
After this, os tests pass in wasi, so add them to the list run by "make tinygo-test-wasi".
Add direct test for the problem to make the fix commit clearer.
Noticed while implementing MkdirTemp on mac; the upstream tests for MkdirTemp fail without this.
