ARC: fix build warning in elf.h

The cast valid since TASK_SIZE * 2 will never actually cause overflow.

|   CC      fs/binfmt_elf.o
| In file included from ../include/linux/elf.h:4:0,
|                  from ../include/linux/module.h:15,
|                  from ../fs/binfmt_elf.c:12:
| ../fs/binfmt_elf.c: In function load_elf_binar:
| ../arch/arc/include/asm/elf.h:57:29: warning: integer overflow in expression [-Woverflow]
|  #define ELF_ET_DYN_BASE  (2 * TASK_SIZE / 3)
|                              ^
| ../fs/binfmt_elf.c:921:16: note: in expansion of macro ELF_ET_DYN_BASE
|     load_bias = ELF_ET_DYN_BASE - vaddr;

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>