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/gojs/errno_test.go

@@ -3,8 +3,6 @@ package gojs
 import (
 	"syscall"
 	"testing"
-
-	"github.com/tetratelabs/wazero/internal/testing/require"
 )
 
 func TestToErrno(t *testing.T) {
@@ -103,8 +101,9 @@ func TestToErrno(t *testing.T) {
 	for _, tt := range tests {
 		tc := tt
 		t.Run(tc.name, func(t *testing.T) {
-			errno := ToErrno(tc.input)
-			require.Equal(t, tc.expected, errno)
+			if errno := ToErrno(tc.input); errno != tc.expected {
+				t.Fatalf("expected %#v but was %#v", tc.expected, errno)
+			}
 		})
 	}
 }