build: Set noarch as default TARGET_ARCH

This will help for debian upgrade

Change-Id: I2991d324b887e340cb676f7de8ea0dda2ea7c050
Forwarded: https://github.com/jerryscript-project/iotjs/pull/rzr
Bug: jerryscript-project#1945
Bug-Debian: https://bugs.debian.org/957364
Origin: https://github.com/TizenTeam/iotjs/tree/sandbox/rzr/cmake/review/master
IoT.js-DCO-1.0-Signed-off-by: Philippe Coval rzr@users.sf.net

CMakeLists.txt

@@ -107,6 +107,10 @@ if(EXPERIMENTAL)
 endif()
 
 # Add arch-dependant flags
+if(NOT DEFINED TARGET_ARCH)
+  message(WARNING "Use generic flags since TARGET_ARCH is not defined")
+  set(TARGET_ARCH "noarch")
+endif()
 if("${TARGET_ARCH}" STREQUAL "arm")
   iotjs_add_compile_flags(-D__arm__ -mthumb -fno-short-enums -mlittle-endian)
 elseif("${TARGET_ARCH}" STREQUAL "i686")