re-add -fvia-C

There are still some fixes required to get the threaded RTS compilable
with the NCG, and apparently there are problems on 32-bit archs too.

rts/Makefile

@@ -325,6 +325,9 @@ GC_HC_OPTS += -optc-O3
 # upd_evacee() assigments get moved before the object copy.
 SRC_CC_OPTS += -fno-strict-aliasing
 
+# Cmm must be compiled via-C for now, because the NCG can't handle loops
+SRC_HC_OPTS += -fvia-C
+
 # We *want* type-checking of hand-written cmm.
 SRC_HC_OPTS += -dcmm-lint