Support building standalone executables

contrib/build_executable.jl
Builds an executable that doesn't require any julia source code.
The user needs to provide a julia script that contains a function
main() taking no argument. The file may contain other needed julia
code, but must otherwise not rely on relative import, using or
include statements. It will be compiled into the sysimg.

Tested on linux. May work on other platforms.

Example:
$ cat hello_world.jl
function main()
    println("Hello world!")
end

$ julia contrib/build_executable.jl standalone_test hello_world.jl
[...]

$ usr/bin/standalone_test
Hello world!