Add a new ".nop" directive to the assembler to allow the creation of no-op instructions in an architeture neutral manner.

* read.c (s_nop): New function.  Handles the .nop directive.
	(potable): Add entry for "nop".
	(s_nops): Code tidy.
	* read.h (s_nop): Add prototype.
	* config/tc-bpf.h (md_single_noop_insn): Define.
	* config/tc-mmix.h (md_single_noop_insn): Define.
	* config/tc-or1k.h (md_single_noop_insn): Define.
	* config/tc-s12z.c (md_assemble): Preserve the input line pointer,
	rather than corrupting it.
	* write.c (relax_segment): Update error message regarding
	non-absolute values passed to .fill and .nops.
	* NEWS: Mention the new directive.
	* doc/as.texi: Document the new directive.
	* doc/internals.texi: Document the new internal macros used to
	implement the new directive.
	* testsuite/gas/all/nop.s: New test.
	* testsuite/gas/all/nop.d: New test control file.
	* testsuite/gas/all/gas.exp: Run the new test.
	* testsuite/gas/elf/dwarf-5-nop-for-line-table.s: New test.
	* testsuite/gas/elf/dwarf-5-nop-for-line-table.d: New test
	control file.
	* testsuite/gas/elf/elf.exp: Run the new test.
	* testsuite/gas/i386/space1.l: Adjust expected output.

gas/read.h

@@ -207,6 +207,7 @@ extern void s_purgem (int);
 extern void s_rept (int);
 extern void s_set (int);
 extern void s_space (int mult);
+extern void s_nop (int);
 extern void s_nops (int);
 extern void s_stab (int what);
 extern void s_struct (int);