sysfs: adds FS.Stat and companions in platform (#1140)

This centralizes filestat logic by making our own `Stat_t` similar to
`syscall.Stat_t`. This exposes utilities in the platform package and
adds a new function `FS.Stat` which avoids having to use `fs.File` to
get the same info. Doing so at the FS abstraction allows us to optimize
how it is implemented internally using portable means (e.g.
`os.StatFile`) or OS-specific means where necessary, e.g. in windows.

This also ensures `platform.OpenFile` returns syscall.Errno and
centralizes error checking with a new `require.EqualErrno` test.

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/sysfs/readfs.go

@@ -5,6 +5,8 @@ import (
 	"io/fs"
 	"os"
 	"syscall"
+
+	"github.com/tetratelabs/wazero/internal/platform"
 )
 
 // NewReadFS is used to mask an existing FS for reads. Notably, this allows
@@ -125,3 +127,8 @@ func maskForReads(f fs.File) fs.File {
 		panic("BUG: unhandled pattern")
 	}
 }
+
+// Stat implements FS.Stat
+func (r *readFS) Stat(path string, stat *platform.Stat_t) error {
+	return r.fs.Stat(path, stat)
+}