x86_64: add -fno-builtin to gcc command line for .hc files.

This seems to be required now that we're stealing more registers.

ghc/compiler/main/DynFlags.hs

@@ -1266,11 +1266,15 @@ machdepCCOpts dflags
 			-- and get in the way of -split-objs.  Another option
 			-- would be to throw them away in the mangler, but this
 			-- is easier.
-		 "-fno-unit-at-a-time"
+		 "-fno-unit-at-a-time",
 			-- unit-at-a-time doesn't do us any good, and screws
 			-- up -split-objs by moving the split markers around.
 			-- It's only turned on with -O2, but put it here just
 			-- in case someone uses -optc-O2.
+		 "-fno-builtin"
+			-- calling builtins like strlen() using the FFI can
+			-- cause gcc to run out of regs, so use the external
+			-- version.
 		] )
 
 #elif mips_TARGET_ARCH