cr: determine the version of the ARM architecture of the target machine

This patch modifies the build system to analyze the version
of the target ARM architecture and setup the compiler flags
accordingly. This makes it possible to support the ARM
architectures prior to ARMv7.

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