makefile: add -DNDEBUG to KBUILD_CFLAGS
makefile: add to -Os -> call cc-disable-warning,maybe-uninitialized
Disable -fconserve-stack on ARM v2

There are reports that -fconserve-stack misaligns variables on the stack.
Disable it for ARM to work around this gcc bug.

v2: Move top level flags definition up
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>