treewide: implement basic aarch64 support

This adds aarch64 platforms, and some fixes for aarch64 support. This is
not yet complete; e.g. toolchains for aarch64 targets are still missing.

I renamed the amd64 platforms to x86_64 and efi to uefi for consistency
with @platforms.

syscall.Dup2 does not exist on arm64, but unix.Dup2 does.

Change-Id: I3ab081b2b852945b723ec83768f79000b8c4def4
Reviewed-on: https://review.monogon.dev/c/monogon/+/4173
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
Tested-by: Jenkins CI
diff --git a/osbase/bringup/bringup.go b/osbase/bringup/bringup.go
index 6373b64..8fcb8fe 100644
--- a/osbase/bringup/bringup.go
+++ b/osbase/bringup/bringup.go
@@ -18,7 +18,6 @@
 	"runtime/debug"
 	"strings"
 	"sync/atomic"
-	"syscall"
 	"time"
 
 	"github.com/opencontainers/runc/libcontainer/cgroups"
@@ -190,7 +189,7 @@
 	var wErr, fErr error
 	wErr = wConn.Control(func(wFd uintptr) {
 		fErr = fConn.Control(func(fFd uintptr) {
-			err = syscall.Dup2(int(wFd), int(fFd))
+			err = unix.Dup2(int(wFd), int(fFd))
 		})
 	})