Allow trailing commas in bare lists

This applies to macro arguments, DB, DW, DL, DS,
PRINT, PRINTLN, EXPORT, PURGE, and OPT.

It also removes support for empty entries in DB/DW/DL.
(Deprecating it would require keeping parser support,
which is ambiguous with trailing commas.)

Fixes #753

test/asm/trailing-commas.err

@@ -0,0 +1,2 @@
+warning: trailing-commas.asm(7): [-Wempty-macro-arg]
+    Empty macro argument