Makefile: sort input filenames to SHA256STAMP

Make's 'wildcard' function returns matching file paths in an
unpredictable order, thereby causing false mismatches in the produced
SHA256STAMPs from system to system. This commit sorts the file paths
given to 'cat' to make the stamps deterministic.

Changelog-Fixed: The build system no longer spuriously regenerates generated sources due to differences in `readdir`(3) sort order.

Makefile

@@ -264,9 +264,9 @@ endif
 # Git doesn't maintain timestamps, so we only regen if sources actually changed:
 # We place the SHA inside some generated files so we can tell if they need updating.
 # Usage: $(call SHA256STAMP_CHANGED)
-SHA256STAMP_CHANGED = [ x"`sed -n 's/.*SHA256STAMP://p' $@ 2>/dev/null`" != x"`cat $(filter-out FORCE,$^) | sha256sum | cut -c1-64`" ]
+SHA256STAMP_CHANGED = [ x"`sed -n 's/.*SHA256STAMP://p' $@ 2>/dev/null`" != x"`cat $(sort $(filter-out FORCE,$^)) | sha256sum | cut -c1-64`" ]
 # Usage: $(call SHA256STAMP,commentprefix)
-SHA256STAMP = echo '$(1) SHA256STAMP:'`cat $(filter-out FORCE,$^) | sha256sum | cut -c1-64` >> $@
+SHA256STAMP = echo '$(1) SHA256STAMP:'`cat $(sort $(filter-out FORCE,$^)) | sha256sum | cut -c1-64` >> $@
 
 # generate-wire.py --page [header|impl] hdrfilename wirename < csv > file
 %_wiregen.h: %_wire.csv $(WIRE_GEN_DEPS)