ARM: nommu: fix warning with checksyscalls.sh

  CALL    scripts/checksyscalls.sh
<stdin>:46:0: warning: "__IGNORE_migrate_pages" redefined [enabled by default]
In file included from <stdin>:2:0:
/home/winfred/git/linux-emcraft/arch/arm/include/asm/unistd.h:474:0: note: this is the location of the previous definition

This is caused because we define __IGNORE_migrate_pages to be 1, but
in the case of nommu, it's defined to be empty.  Fix this by just
defining the __IGNORE_ symbols to be empty.