Fix Makefile to detect 64bits architectures

uname -a could return 64 bits even in a 32 bits OS, if the OS is within
a docker or chroot hosted on a 64bits OS

Makefile

 UNAME := $(shell uname)
+ARCH := $(shell getconf LONG_BIT)
 CC=gcc
 exec = buildsupport
 
@@ -26,7 +27,7 @@ ifeq ($(UNAME), Linux)
 		fi ;                                    \
 	fi
 endif
-	uname -a | grep x86_64 && EXTRAFLAG="--target=x86_64-linux" ; \
+	[ $(ARCH) == 64 ] && EXTRAFLAG="--target=x86_64-linux" ; \
 	ADA_PROJECT_PATH=`ocarina-config --prefix`/lib/gnat:$$ADA_PROJECT_PATH \
             $(gnatpath)gprbuild -x -g $(exec) -p -P buildsupport.gpr -XBUILD="debug" $$EXTRAFLAG