[testsuite] Add effective target ident_directive

[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.