[RFC] CMake: Set custom RelWithDebInfo compile flags.

Follow-up from #1366. Tested that it uses the correct flags with something like:

```
$ export PATH=$HOME/bin:$PATH
$ cat > $HOME/bin/cc
#!/bin/bash
echo "$*" >> $HOME/debugcc
/usr/bin/gcc $*
$ make CMAKE_BUILD_TYPE=NvimReleaseWithDebInfob
$ less $HOME/debugcc
Yes, to keep assertions (https://github.com/neovim/neovim/pull/1366#issuecomment-61456353 / https://github.com/neovim/neovim/pull/1366#issuecomment-61464073)