Makefile: Avoid non-portable GNU sed extension

Use [:space:] instead of \s and \S in regular expression that
determines the sandbox target architecture.  Fixes the build
failure on OpenBSD introduced with commit 4e65ca00
("efi_loader: bootmgr: add booting from removable media").

Fixes: f7691a6d ("sandbox: allow cross-compiling sandbox")
Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>