UAPI_HEADER_TEST and CC_CAN_LINK dependency

[Nick Desaulniers]

Hello Masahiro,

We'd like to be able to better test UAPI headers via
CONFIG_UAPI_HEAD_TEST=y with clang builds during cross compiling.
We're finding that CC_CAN_LINK=y is not enabled because clang (unlike
gcc) isn't configured with a cross libc in hand, and one may not be
available in the environment. This is a similar constraint to the
prebuilt GCC images distributed on kernel.org.

Looking at
commit d6fc9fcbaa65 ("kbuild: compile-test exported headers to ensure
they are self-contained")

Simply removing the dependency on CC_CAN_LINK=y allows us to run the
UAPI header tests, which pass on ARCH=arm64 and native x86_64 builds.
https://android-review.googlesource.com/c/kernel/common/+/1991156

I assume there may be some additional constraints perhaps for other
architectures or certain configs?

Another suggestion from the bionic maintainer to ensure the tests are
more hermetic/self-contained would be to add -ffreestanding to the
compiler invocations of these tests.

Do you have thoughts on how we might be able to eliminate the
dependency on CC_CAN_LINK for UAPI_HEADER_TEST? Otherwise, I suspect
we will have to use `USERCFLAGS=--sysroot=/path/to/sysroot` to make
CC_CAN_LINK work for clang cross compilation. See also:
commit f67695c9962e ("kbuild: Add environment variables for userprogs flags")

-- 
Thanks,
~Nick Desaulniers