Makefile: avoid non-standard sed expressions

The '?' that is used in the sed expression that determines MK_ARCH
is part of the extended regular expression syntax, but the default
behaviour of sed as specified by POSIX is to use only basic regular
expression syntax.  Use the equivalent '\{0,1\}' instead as
suggested by the OpenBSD re_format(7) man page.

Fixes: 04b1d842 ("Makefile: fix empty MK_ARCH when using ccache")
Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Tested-by: Quentin Schulz <quentin.schulz@cherry.de>