Make -march settings part of USERCFLAGS

The current -march settings will cause an error if the user a
conflicting flags such as -mcpu=cortex-a15. Use USERCFLAGS instead
of CFLAGS so that a default is provided but can be overridden.

Change-Id: I8ffecd6bfd7ad5794486a5d14d27bce3170898d3

Acked-by: Alexander Kartashov <alekskartashov@parallels.com>

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>