memblock tests: Fix compilation error.

memblock tests: Fix compilation error.

Commit cf4694be2b2cf ("tools: Add atomic_test_and_set_bit()") changed
tools/arch/x86/include/asm/atomic.h to include <asm/asm.h>, which causes
'make -C tools/testing/memblock' to fail with:

In file included from ../../include/asm/atomic.h:6,
                 from ../../include/linux/atomic.h:5,
                 from ./linux/mmzone.h:5,
                 from ../../include/linux/mm.h:5,
                 from ../../include/linux/pfn.h:5,
                 from ./linux/memory_hotplug.h:6,
                 from ./linux/init.h:7,
                 from ./linux/memblock.h:11,
                 from tests/common.h:8,
                 from tests/basic_api.h:5,
                 from main.c:2:
../../include/asm/../../arch/x86/include/asm/atomic.h:11:10: fatal error: asm/asm.h: No such file or directory
   11 | #include <asm/asm.h>
      |          ^~~~~~~~~~~

Create a symlink to asm/asm.h in the same manner as the existing one to
asm/cmpxchg.h.

Signed-off-by: Aaron Thompson <dev@aaront.org>
Link: https://lore.kernel.org/r/010101857c402765-96e2dbc6-b82b-47e2-a437-4834dbe0b96b-000000@us-west-2.amazonses.com
Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>