d: Disable the D runtime garbage collector after initializing (PR103520)

d: Disable the D runtime garbage collector after initializing (PR103520)

Not all targets that support building libdruntime have a stable garbage
collector, so to avoid running into problems where live memory allocated
by the D GC is freed, disable all in-flight collections until a time
when scanning is more reliably implemented everywhere.

	PR d/103520

gcc/d/ChangeLog:

	* d-frontend.h (gc_disable): Declare.
	* d-lang.cc (d_init_options): Disable the D runtime garbage collector
	after initializing.