For many architectures (armv6, aarch64), is -mcpu is essential for proper
detection of  CPU features. Follow compile.test case and reuse CXXFLAGS 
from make environment also in arch.test.

--- config.tests/unix/arch.test.orig	2017-02-05 07:43:57 UTC
+++ config.tests/unix/arch.test
@@ -17,7 +17,7 @@ else
 fi
 
 LFLAGS="$SYSROOT_FLAG"
-CXXFLAGS="$SYSROOT_FLAG"
+CXXFLAGS="$CXXFLAGS $SYSROOT_FLAG"
 
 while [ "$#" -gt 0 ]; do
     PARAM=$1