libphobos: libdruntime doesn't support shadow stack (PR95680)

libphobos: libdruntime doesn't support shadow stack (PR95680)

The first implementation hit a front-end implementation bug where
version conditions are resolved ahead of static if confitions.

The logic for whether to use asm implemented fiber_switchContext or
libc's swapcontext has been moved from GNU_Enable_CET to version CET.

libphobos/ChangeLog:

	PR d/95680
	PR d/97007
	* Makefile.am (AM_MAKEFLAGS): Remove $(CET_FLAGS).
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac (DCFG_ENABLE_CET): Remove substitution.
	(CET_DFLAGS): Substitute.
	* libdruntime/Makefile.am (AM_DFLAGS): Add $(CET_DFLAGS).
	(AM_CFLAGS): Add $(CET_FLAGS).
	(AM_CCASFLAGS): Likewise.
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/core/thread.d: Replace static if GNU_Enable_CET
	condition with `version (CET)'.
	* libdruntime/gcc/config.d.in (GNU_Enable_CET): Remove.
	* src/Makefile.am (AM_DFLAGS): Add $(CET_DFLAGS).
	(AM_CFLAGS): Add $(CET_FLAGS).
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/testsuite_flags.in: Add $(CET_DFLAGS) to --gdcflags.