kbuild: skip install/check of headers right under uapi directories

Since commit 61562f98 ("uapi: export all arch specifics
directories"), "make INSTALL_HDR_PATH=$root/usr headers_install"
deletes standard glibc headers and others in $(root)/usr/include.

The cause of the issue is that headers_install now starts descending
from arch/$(hdr-arch)/include/uapi with $(root)/usr/include for its
destination when installing asm headers.  So, headers already there
are assumed to be unwanted.

When headers_install starts descending from include/uapi with
$(root)/usr/include for its destination, it works around the problem
by creating an dummy destination $(root)/usr/include/uapi, but this
is tricky.

To fix the problem in a clean way is to skip headers install/check
in include/uapi and arch/$(hdr-arch)/include/uapi because we know
there are only sub-directories in uapi directories.  A good side
effect is the empty destination $(root)/usr/include/uapi will go
away.

I am also removing the trailing slash in the headers_check target to
skip checking in arch/$(hdr-arch)/include/uapi.

Fixes: 61562f98

 ("uapi: export all arch specifics directories")
Reported-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>