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>