[MIPS] Rename CONFIG_BUILD_ELF64 into KBUILD_64BIT_SYM32

This patch renames it for 3 reasons:

    - "CONFIG" pattern is used by Kconfig. Now this macro is
      no more defined by Kconfig but by Kbuild itself make this
      clear by translating "CONFIG" into "KBUILD".

    - "ELF32" word is improper because it is irrelevant to ELF
      format and it makes confusion with CONFIG_BOOT_ELF32. So
      translate it with SYM32.

    - Add "64BIT" part to make clear that this macro implies a
      64 bits kernel.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>