[testsuite] Add effective target ident_directive

On nvptx we run into:
...
FAIL: c-c++-common/ident-1b.c  -Wc++-compat   scan-assembler GCC:
FAIL: c-c++-common/ident-2b.c  -Wc++-compat   scan-assembler GCC:
...

Using a scan-assembler directive adds -fno-indent to the compile options.
The test c-c++-common/ident-1b.c adds dg-options "-fident", and intends to
check that the -fident overrides the -fno-indent, by means of the
scan-assembler.  But for nvptx, there's no .ident directive, both with -fident
and -fno-ident.

Fix this by adding an effective target ident_directive, and requiring
it in both test-cases.

Tested on nvptx and x86_64.

gcc/testsuite/ChangeLog:

2020-09-24  Tom de Vries  <tdevries@suse.de>

	* lib/target-supports.exp (check_effective_target_ident_directive): New proc.
	* c-c++-common/ident-1b.c: Require effective target ident_directive.
	* c-c++-common/ident-2b.c: Same.

gcc/testsuite/c-c++-common/ident-1b.c

@@ -2,6 +2,7 @@
  * Make sure scan-assembler turns off .ident unless -fident in testcase */
 /* { dg-do compile } */
 /* { dg-options "-fident" } */
+/* { dg-require-effective-target ident_directive }*/
 int i;
 
 /* { dg-final { scan-assembler "GCC: " { xfail { { hppa*-*-hpux* && { ! lp64 } } || { powerpc-ibm-aix* || powerpc*-*-darwin* } } } } } */

gcc/testsuite/c-c++-common/ident-2b.c

@@ -2,6 +2,7 @@
  * Make sure scan-assembler-times turns off .ident unless -fident in testcase */
 /* { dg-do compile } */
 /* { dg-options "-fident" } */
+/* { dg-require-effective-target ident_directive }*/
 int ident;
 
 /* { dg-final { scan-assembler "GCC: " { xfail { { hppa*-*-hpux* && { ! lp64 } } || { powerpc-ibm-aix* || powerpc*-*-darwin* } } } } } */

gcc/testsuite/lib/target-supports.exp

@@ -10510,3 +10510,12 @@ proc check_symver_available { } {
 	    }
 	}]
 }
+
+# Return 1 if emitted assembly contains .ident directive.
+
+proc check_effective_target_ident_directive {} {
+    return [check_no_messages_and_pattern ident_directive \
+		"(?n)^\[\t\]+\\.ident" assembly {
+	int i;
+    }]
+}