libphobos: Push all callee-save registers on the stack before GC scan

libphobos: Push all callee-save registers on the stack before GC scan

This is the library fix for PR103520 that also prevents the garbage
collector from releasing live memory.  However this requires that the
host compiler has been patched with this fix, so the GC will remain
disabled in the D front-end for now until enough time has passed for
this to have trickled down into enough releases.

libphobos/ChangeLog:

	* libdruntime/core/thread.d (callWithStackShell): Push all callee-save
	registers on the stack using inline assembly.

(cherry picked from commit 2fb6a57f9ffa9ac964931b439b6d1600a698e377)