ext-tool.mk: fix SUPPORT_LIB_DIR calculation for toolchains without libstdc++

SUPPORT_LIB_DIR would get resolved to the main buildroot directory for
external toolchains without C++ support, as:

 - gcc -print-file-name=<nonexisting-file> returns <nonexisting-file>
 - readlink -f <nonexisting-file> returns $PWD/<nonexisting-file>

So fix it by ensuring output of gcc -print-file-name actually exists
before using it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/toolchain-external/ext-tool.mk

@@ -384,11 +384,12 @@ $(STAMP_DIR)/ext-toolchain-installed: $(STAMP_DIR)/ext-toolchain-checked
 	ARCH_LIBC_A_LOCATION=`readlink -f $$(LANG=C $(TOOLCHAIN_EXTERNAL_CC) $(TOOLCHAIN_EXTERNAL_CFLAGS) -print-file-name=libc.a)` ; \
 	ARCH_SYSROOT_DIR=`echo $${ARCH_LIBC_A_LOCATION} | sed -r -e 's:usr/lib(64)?/(.*/)?libc\.a::'` ; \
 	ARCH_LIB_DIR=`echo $${ARCH_LIBC_A_LOCATION} | sed -r -e 's:.*/usr/(lib(64)?)/(.*/)?libc.a:\1:'` ; \
+	SUPPORT_LIB_DIR="" ; \
 	if test `find $${ARCH_SYSROOT_DIR} -name 'libstdc++.a' | wc -l` -eq 0 ; then \
-		LIBSTDCPP_A_LOCATION=`readlink -f $$(LANG=C $(TOOLCHAIN_EXTERNAL_CC) $(TOOLCHAIN_EXTERNAL_CFLAGS) -print-file-name=libstdc++.a)` ; \
-		SUPPORT_LIB_DIR=`echo $${LIBSTDCPP_A_LOCATION} | sed -r -e 's:libstdc\+\+\.a::'` ; \
-	else \
-		SUPPORT_LIB_DIR="" ; \
+		LIBSTDCPP_A_LOCATION=$$(LANG=C $(TOOLCHAIN_EXTERNAL_CC) $(TOOLCHAIN_EXTERNAL_CFLAGS) -print-file-name=libstdc++.a) ; \
+		if [ -e "$${LIBSTDCPP_A_LOCATION}" ]; then \
+			SUPPORT_LIB_DIR=`readlink -f $${LIBSTDCPP_A_LOCATION} | sed -r -e 's:libstdc\+\+\.a::'` ; \
+		fi ; \
 	fi ; \
 	ARCH_SUBDIR=`echo $${ARCH_SYSROOT_DIR} | sed -r -e "s:^$${SYSROOT_DIR}(.*)/$$:\1:"` ; \
 	mkdir -p $(TARGET_DIR)/lib ; \