arch/arch.mk: fix check-package warnings

"make check-package" is not happy with the formatting of the recently
introduced arch.mk:

arch/arch.mk:1: should be 80 hashes (http://nightly.buildroot.org/#writing-rules-mk)
arch/arch.mk:2: should be 1 hash (http://nightly.buildroot.org/#writing-rules-mk)
arch/arch.mk:4: should be 1 hash (http://nightly.buildroot.org/#writing-rules-mk)
arch/arch.mk:5: should be 80 hashes (http://nightly.buildroot.org/#writing-rules-mk)
arch/arch.mk:6: should be a blank line (http://nightly.buildroot.org/#writing-rules-mk)

Let's fix this by adding a comment header that makes check-package
happy.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

arch/arch.mk

@@ -1,3 +1,9 @@
+################################################################################
+#
+# Architecture-specific definitions
+#
+################################################################################
+
 # Allow GCC target configuration settings to be optionally
 # overwritten by architecture specific makefiles.