gcc-package-sdk.inc: Allow executable extension to be overridden

gcc-package-sdk.inc: Allow executable extension to be overridden

On platforms like windows, executables have extensions. Whilst I'm not proposing
we wholesale support windows extensions, this small tweak allows a cross compiler
targetting mingw to be built which does seem like a good use case.

The patch therefore adds an EXEEXT which the mingw layer can set for the libexec
symlinks.

(From OE-Core rev: 8dcf0d95b654fa6cc56193168aaa744052ad8ffc)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>