A few files are compiled with gcc regardless of setting CC

This file seems to do some guesswork, override it with CC if set

Bug: https://bugs.gentoo.org/734302

--- a/kBuild/tools/GCC3.kmk
+++ b/kBuild/tools/GCC3.kmk
@@ -84,6 +84,14 @@
  TOOL_GCC3_CXX += -fmessage-length=0
 endif
 
+ifneq ($(CC),)
+	TOOL_GCC3_CC = $(CC)
+	TOOL_GCC3_LD = $(CC)
+endif
+ifneq ($(AR),)
+	TOOL_GCC3_AR = $(AR)
+endif
+
 # General Properties used by kBuild
 TOOL_GCC3_COBJSUFF         ?= .o
 TOOL_GCC3_CFLAGS           ?=