gas: improve unrecognized command line option diagnostic

Printing optc with %c makes sense only when optc is actually a
character. Add logic to also deal with unrecognized long options,
rejected by md_parse_option() rather than get_opt_long_only(). Also
quote the reproduced strings, such that possible included whitespace
can be recognized.
diff --git a/gas/as.c b/gas/as.c
index 259dc0e..28aa4e3 100644
--- a/gas/as.c
+++ b/gas/as.c
@@ -666,8 +666,12 @@
 		verbose = 1;
 	      break;
 	    }
+	  else if (is_a_char (optc))
+	    as_bad (_("unrecognized option `-%c%s'"), optc, optarg ? optarg : "");
+	  else if (optarg)
+	    as_bad (_("unrecognized option `--%s=%s'"), longopts[longind].name, optarg);
 	  else
-	    as_bad (_("unrecognized option -%c%s"), optc, optarg ? optarg : "");
+	    as_bad (_("unrecognized option `--%s'"), longopts[longind].name);
 	  /* Fall through.  */
 
 	case '?':
diff --git a/gas/testsuite/gas/arm/armv2-mp-bad.l b/gas/testsuite/gas/arm/armv2-mp-bad.l
index eb97135..d0159b0 100644
--- a/gas/testsuite/gas/arm/armv2-mp-bad.l
+++ b/gas/testsuite/gas/arm/armv2-mp-bad.l
@@ -1,3 +1,3 @@
 Assembler messages:
 [^:]*: extension does not apply to the base architecture
-[^:]*: unrecognized option -march=armv2\+mp
+[^:]*: unrecognized option .*-march=armv2\+mp.*
diff --git a/gas/testsuite/gas/arm/dotprod-legacy-arch.l b/gas/testsuite/gas/arm/dotprod-legacy-arch.l
index a627a36..7bbdf63 100644
--- a/gas/testsuite/gas/arm/dotprod-legacy-arch.l
+++ b/gas/testsuite/gas/arm/dotprod-legacy-arch.l
@@ -1,3 +1,3 @@
 Assembler messages:
 [^:]*: extension does not apply to the base architecture
-[^:]*: unrecognized option -march=armv8.1-a\+dotprod
+[^:]*: unrecognized option .*-march=armv8\.1-a\+dotprod.*
diff --git a/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l b/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l
index 76208d2..a6fc3dc 100644
--- a/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l
+++ b/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l
@@ -1,3 +1,3 @@
 Assembler messages:
 [^:]*: extension does not apply to the base architecture
-[^:]*: unrecognized option -march=armv7\+idiv
+[^:]*: unrecognized option .*-march=armv7\+idiv.*