build: Stop linking morestack.S to rt

__morestack has to be linked statically. The odd calling convention on x86_64
results in the arguments to __morestack (r10 & r11) being clobbered by the
dynamic linker. gcc links all the __morestack functions statically.

mk/rt.mk

@@ -67,7 +67,6 @@ RUNTIME_CS_$(1) := \
 
 RUNTIME_S_$(1) := rt/arch/$$(HOST_$(1))/_context.S \
                   rt/arch/$$(HOST_$(1))/ccall.S \
-                  rt/arch/$$(HOST_$(1))/morestack.S \
                   rt/arch/$$(HOST_$(1))/record_sp.S
 
 RUNTIME_HDR_$(1) := rt/globals.h \

src/rt/rustrt.def.in

@@ -1,4 +1,3 @@
-__morestack
 chan_id_send
 check_claims
 debug_box