rtems/config/tools: Add checks to stdcxx.py install script

Summary

As noted in issue #29 (closed), the build fails when run on a fresh toolchain. This occurs for 2 reasons:

1. The directory to which the script is to be installed does not exist (this only happens if this is the first toolchain being built to that prefix).
2. arch-rtems6-gcc has not yet been installed, due to which one of the macros is left defined wrong. This is a result of not testing on a completely fresh toolchain after altering the order of some files in !35 (merged)

To circumvent these errors, two checks have been added:

1. A check to see if the install path exists first, which if not, will be created and the script (stdcxx.py) will be placed there.
2. The gcc_version macro is used as backup if the toolchain's GCC is not yet built (in the case of fresh builds)