Add a "reverse AUX hack" for ARM.

MIPS and x86 are almost completely switched over to uapi-only. ARM is still
currently old-only. A cleanup of the uapi headers over the weekend means
that the uapi-only <linux/kexec.h> is now in the right place, which will break
the ARM build unless the ARM build also looks in the uapi headers if it can't
find an old header.

Change-Id: Ie7a53ef2122b43cdef64b71d7b36a8381992a461