Fix some build errors on arm64.

Initialize the VDSO "os" and "arch" fields explicitly,
or the VDSO load process would failed on arm64 platform.

Signed-off-by: Haibo Xu <haibo.xu@arm.com>
Change-Id: Ic6768df88e43cd7c7956eb630511672ae11ac52f

pkg/sentry/kernel/ptrace_arm64.go

@@ -17,7 +17,6 @@
 package kernel
 
 import (
-	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserror"
 )

pkg/sentry/loader/vdso.go

@@ -268,6 +268,8 @@ func PrepareVDSO(ctx context.Context, mfp pgalloc.MemoryFileProvider) (*VDSO, er
 		// some applications may not be able to handle multiple [vdso]
 		// hints.
 		vdso:  mm.NewSpecialMappable("", mfp, vdso),
+		os:    info.os,
+		arch:  info.arch,
 		phdrs: info.phdrs,
 	}, nil
 }