d/i386: Add CET TargetInfo key and predefined version [PR118654]

Adds a new i386 d_target_info_spec entry to handle requests for
`__traits(getTargetInfo, "CET")', and add predefined target version
`GNU_CET' when the option `-fcf-protecton' is used.

Both TargetInfo key and predefined version have been added to the D
front-end documentation.

In the library, `GNU_CET' replaces the existing use of the user-defined
version flag `CET' when building libphobos.

	PR d/118654

gcc/ChangeLog:

	* config/i386/i386-d.cc (ix86_d_target_versions): Predefine GNU_CET.
	(ix86_d_handle_target_cf_protection): New.
	(ix86_d_register_target_info): Add 'CET' TargetInfo key.

gcc/d/ChangeLog:

	* implement-d.texi: Document CET version and traits key.

libphobos/ChangeLog:

	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Remove CET_DFLAGS.
	* libdruntime/Makefile.am: Replace CET_DFLAGS with CET_FLAGS.
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/core/thread/fiber/package.d: Replace CET with GNU_CET.
	* src/Makefile.am: Replace CET_DFLAGS with CET_FLAGS.
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/testsuite_flags.in: Replace CET_DFLAGS with CET_FLAGS.

gcc/testsuite/ChangeLog:

	* gdc.dg/target/i386/i386.exp: New test.
	* gdc.dg/target/i386/targetinfo_CET.d: New test.