A garbage collector with precise heap scanning can now be selected

Precise heap scanning can now be enabled by a DRT option to the
$(LINK2 $(ROOT_DIR)spec/garbage.html, GC configuration),
e.g. by passing `--DRT-gcopt=gc:precise` on the command
line or by redefining `rt_options`. See the
$(LINK2 $(ROOT_DIR)spec/garbage.html#precise_gc, documentation)
for details.
