[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
Oops, copied the wrong line out of `ccmake`. Sorry, fixed now.