Allow weblist to work even if assembly is not available
Weblist provides source and assembly combined in a web document.
It needs access to the binary to print the assembly, but currently
refuses to generate the source if the binary can't be find.
Fall back to just generating the source if the binary isn't found.
Add source_path option to point pprof to source files
Currently pprof will look for source files only on the current directory
and its parents. This makes it hard to examine sources on jobs where
there are multiple source trees (eg from different libraries).
Add a variable to provide a search path for source files. It will default
to the cwd, so there will be no change in behavior by default.