Use $CC from environment (Clang)

Tuprules.tup

@@ -5,6 +5,20 @@ export DHALL_GENODE_TYPES
 export DHALL_PRELUDE
 export PKG_CONFIG_PATH
 
+export CC
+export CXX
+export LD
+export OBJCOPY
+export AR
+export AS
+
+CC = $CC
+CXX= $CXX
+LD = $LD
+OBJCOPY = $OBJCOPY
+AR = $AR
+AS = $AS
+
 ERRATA_DIR=$(TUP_CWD)/errata
 
 OUTPUTS_OUT=$(TUP_CWD)/out
@@ -20,7 +34,6 @@ AS_MARCH = -march=i686 --32
 endif
 
 ifeq (@(TUP_ARCH),x86_64)
-TOOL_CHAIN_PREFIX=genode-x86-
 CC_MARCH = -m64 -mcmodel=large
 LD_MARCH = -melf_x86_64
 endif
@@ -83,12 +96,6 @@ RAW_DIR = $(OUT_RAW_DIR)/$(RAW_NAME)/$(DEPOT_VERSION)
 	cp %f %o \
 |> $(OUT_RAW_DIR)/$(RAW_NAME)/%b $(OUTPUTS_OUT)/<raw>
 
-CC = $(TOOL_CHAIN_PREFIX)gcc
-CXX = $(TOOL_CHAIN_PREFIX)g++
-LD= $(TOOL_CHAIN_PREFIX)ld
-AR = $(TOOL_CHAIN_PREFIX)ar
-AS = $(TOOL_CHAIN_PREFIX)as
-OBJCOPY = $(TOOL_CHAIN_PREFIX)objcopy
 AS_OPT += $(AS_MARCH)
 
 LIBGCC = `$(CC) $(CC_MARCH) -print-libgcc-file-name`