selftests/mm: pass down full CC and CFLAGS to check_config.sh

check_config.sh checks that liburing is available by running the compiler
provided as its first argument. This makes two assumptions:

1. CC consists of only one word
2. No extra flag is required

Unfortunately, there are many situations where these assumptions don't
hold. For instance:

- When using Clang, CC consists of multiple words
- When cross-compiling, extra flags may be required to allow the
compiler to find headers

Remove these assumptions by passing down CC and CFLAGS as-is from the
Makefile, so that the same command line is used as when actually building
the tests.

Link: https://lkml.kernel.org/r/20260122170224.4056513-4-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Usama Anjum <Usama.Anjum@arm.com>
Cc: wang lian <lianux.mm@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>