Change L1_CACHE_BYTES to 16 bytes.
Tested for 16 days on rp3440.

Additional remarks from Helge Deller:
Saves ~17 kb of kernel code/data and gives a slight performance improvement in
various test cases.
Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>