alpha: Use symbol assignment in SYM_END

Alpha uses the .set directive to select assembler options, so the
symbol assignment emitted by the generic SYM_END macro fails to assemble.
This breaks COMPILE_TEST builds left and right, e.g.:

  Error: junk at end of line, first unrecognized character is `,'
  Error: .size expression for bcm6846_runner_start does not evaluate to a constant

Override SYM_END to use an ordinary symbol assignment for the calculated
size. Preserve the symbol type, size and local size symbol while leaving
other architectures unchanged.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202611031620.n4ptlpo7-lkp@intel.com/
Signed-off-by: Linus Walleij <linusw@kernel.org>