alpha: use L1_CACHE_BYTES for cacheline size in the linker script

Currently the linker script uses 64 for cacheline size which isn't
optimal for all cases.  Include asm/cache.h and use L1_CACHE_BYTES
instead as suggested by Sam Ravnborg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sam Ravnborg <sam@ravnborg.org>