* ldlex.l: ASSERT is recognized in SCRIPT env. NAMES cannot

contain commas in EXP env.
	* ldgram.y (extern_name_list): Push to EXP env, move body to ...
	(extern_name_list_body): ... here.
	(script_file, ifile_list): Reformat.
	(statement): Add ASSERT.
testsuite:
	* ld-scripts/assert.t: Add additional cases.
	* ld-scripts/extern.t, ld-scripts/extern.s,
	ld-scripts/extern.exp: New.

ld/testsuite/ld-scripts/assert.t

@@ -1,5 +1,9 @@
 SECTIONS
 {
-  .empty : {}
+  .empty : {
+  here = !.;
+  ASSERT (!., "dot is not zero");
+  ASSERT (here, "here is zero");
+  }
   ASSERT (!SIZEOF(.empty), "Empty is not empty")
 }