Avoid redefining CACHE_LINE_SIZE if system provides it.

Avoid redefining CACHE_LINE_SIZE if system provides it.

FreeBSD 14.3 complains about our trying to redefine
the CACHE_LINE_SIZE, which happens to be 1 << 6 == 64
on amd64 but not arm64.

Just check and only #define unless it's already:

../../../lib/ext2fs/unix_io.c:563: warning: "CACHE_LINE_SIZE" redefined
  563 | #define CACHE_LINE_SIZE         64
      |
In file included from /usr/include/sys/param.h:163,
                 from /usr/include/bsm/audit.h:41,
                 from /usr/include/sys/ucred.h:42,
                 from /usr/include/sys/mount.h:37,
                 from ../../../lib/ext2fs/unix_io.c:53:
/usr/include/machine/param.h:92: note: this is the location of the previous definition
   92 | #define CACHE_LINE_SIZE         (1 << CACHE_LINE_SHIFT)
      |

On FreeBSD 14.3, machine/param.h
- for arm64 and powerpc define CACHE_LINE_SIZE to (128),
- for i386, amd64, arm (32) and riscv define (64).

Signed-off-by: Matthias Andree <matthias.andree@gmx.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>