arm: fix a double-definition error of _start symbol

The symbol "_start" is defined twice in arch/arm/lib/vectors.S:
around line 48 and line 54.

If CONFIG_SYS_DV_NOR_BOOT_CFG is defined (as on calimain board),
build fails:

  arch/arm/lib/vectors.S: Assembler messages:
  arch/arm/lib/vectors.S:54: Error: symbol `_start' is already defined
  make[1]: *** [arch/arm/lib/vectors.o] Error 1
  make: *** [arch/arm/lib] Error 2

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>