ARC: [mm] Make stack/heap Non-executable by default

1. Makes this security feature ON by default rather than OFF before.

2. Applications can use mprotect() to selectively override this.

3. ELF binaries have a GNU_STACK segment which can easily override the
   kernel default permissions.
   For nested-functions/trampolines, gcc already auto-enables executable
   stack in elf. Others needing this can use -Wl,--execstack option.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
ARC: gcc 4.8 (-marclinux not needed)

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
ARC: Makefile refactor gcc version specific handling

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