sourcery-codebench: fix installation of manpages

Sourcery CodeBench manpages are installed under
share/doc/<target-triplet>/man/. Add symlinks so that the manpages
become available to "man".

NOTE: I use symlinks instead of moving the manpages, because I think
it is best to do as little as possible to prebuilt packages.
Merge pull request #573 from bjornfor/sourcery-toolchain-manpages

sourcery-codebench: fix installation of manpages