src/{syscall, os}: add File.Stat, with smoke test

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.

src/os/file.go

@@ -193,11 +193,6 @@ func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
 	return f.handle.Seek(offset, whence)
 }
 
-// Stat is a stub, not yet implemented
-func (f *File) Stat() (FileInfo, error) {
-	return nil, &PathError{"stat", f.name, ErrNotImplemented}
-}
-
 func (f *File) SyscallConn() (syscall.RawConn, error) {
 	return nil, ErrNotImplemented
 }