Fix building of llvm's unwind if gcc has been also built

when building gcc an unwind.h header is generate in the cc_tool directory
which is included in the CFLAGS before the path where the llvm's unwind.h file
lives

Reviewed by:	emaste
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D7357

gnu/lib/libgcc/Makefile

@@ -30,8 +30,7 @@ CFLAGS+=	-DTARGET_ARM_EABI
 CFLAGS+=	-DIN_GCC -DIN_LIBGCC2 -D__GCC_FLOAT_NOT_NEEDED \
 		-DHAVE_GTHR_DEFAULT \
 		-I${GCCLIB}/include \
-		-I${GCCDIR}/config -I${GCCDIR} -I. \
-		-I${CCDIR}/cc_tools
+		-I${GCCDIR}/config -I${GCCDIR} -I.
 
 LDFLAGS+=	-nodefaultlibs
 LIBADD+=	c
@@ -102,6 +101,12 @@ CFLAGS+=	-I${SRCTOP}/contrib/libc++/include
 
 .else # MK_LLVM_LIBUNWIND
 
+#
+# Prevent the potential unwind.h from gcc (generated)
+# to be in the include path before the one from llvm unwind
+#
+CFLAGS+=	-I${CCDIR}/cc_tools
+
 .if ${TARGET_CPUARCH} == "arm"
 LIB2ADDEH =	unwind-arm.c libunwind-arm.S pr-support.c unwind-c.c
 .else