Properly export CC variable in Makefile

We cannot use ?= because CC is set by default to "cc". Therefore
CC was never set. In addition we need to export the variable
in order to have the rust cc crate pick it up. Otherwise it is
only used by openlibm where it is explicitly passed.

Makefile

@@ -10,11 +10,11 @@ ifneq ($(TARGET),)
 endif
 
 ifeq ($(TARGET),aarch64-unknown-linux-gnu)
-	CC?=aarch64-linux-gnu-gcc
+	export CC=aarch64-linux-gnu-gcc
 endif
 
 ifeq ($(TARGET),x86_64-unknown-redox)
-	CC?=x86_64-unknown-redox-gcc
+	export CC=x86_64-unknown-redox-gcc
 endif
 
 SRC=\