added cross compiler examples for 32bit and 64bit ARM

Makefile.rule

@@ -25,9 +25,20 @@ VERSION = 0.2.10.rc1
 # FC = gfortran
 
 # Even you can specify cross compiler. Meanwhile, please set HOSTCC.
+
+# cross compiler for Windows
 # CC = x86_64-w64-mingw32-gcc
 # FC = x86_64-w64-mingw32-gfortran
 
+# cross compiler for 32bit ARM
+# CC = arm-linux-gnueabihf-gcc
+# FC = arm-linux-gnueabihf-gfortran
+
+# cross compiler for 64bit ARM
+# CC = aarch64-linux-gnu-gcc
+# FC = aarch64-linux-gnu-gfortran
+
+
 # If you use the cross compiler, please set this host compiler.
 # HOSTCC = gcc