[syserror] Split usermem package

Split usermem package to help remove syserror dependency in go_marshal.
New hostarch package contains code not dependent on syserror.

PiperOrigin-RevId: 365651233

pkg/sentry/fsimpl/testutil/BUILD

@@ -15,6 +15,7 @@ go_library(
         "//pkg/context",
         "//pkg/cpuid",
         "//pkg/fspath",
+        "//pkg/hostarch",
         "//pkg/memutil",
         "//pkg/sentry/fsbridge",
         "//pkg/sentry/fsimpl/tmpfs",

pkg/sentry/fsimpl/testutil/testutil.go

@@ -30,6 +30,8 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/usermem"
+
+	"gvisor.dev/gvisor/pkg/hostarch"
 )
 
 // System represents the context for a single test.
@@ -105,7 +107,7 @@ func (s *System) Destroy() {
 
 // ReadToEnd reads the contents of fd until EOF to a string.
 func (s *System) ReadToEnd(fd *vfs.FileDescription) (string, error) {
-	buf := make([]byte, usermem.PageSize)
+	buf := make([]byte, hostarch.PageSize)
 	bufIOSeq := usermem.BytesIOSequence(buf)
 	opts := vfs.ReadOptions{}