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

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

Rather than implementing support within D runtime itself, use libc
getcontext/swapcontext functions if CET is enabled.

Removes whatever CET support was in the switchContext routine for x86
D runtime, along with setting version AsmExternal, so that the fallback
ucontext_t implementation is used, which is capable of doing shadow
stack handling.

libphobos/ChangeLog:

	PR d/95680
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac (DCFG_ENABLE_CET): Substitute.
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/config/x86/switchcontext.S: Remove CET support code.
	* libdruntime/core/thread.d: Import gcc.config.  Don't set version
	AsmExternal when GNU_Enable_CET is true.
	* libdruntime/gcc/config.d.in (GNU_Enable_CET): Define.
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.