In the flang bash script, we need to be careful _when_ to use <output>
from flang -c -o <output> <input> when generating the relocatable
output file name.
In particular, we should use it in this case:
compilation only flang -c -o <output> <input>
but leave it for the final executable here:
compile, assemble and link flang -o <output> <input>
This change is implemented in get_relocatable_name.
I've also taken the liberty to fix how errors from sub-commands are
reported (without this change, flang always returns 0 on failure).
This is implemented in main.
I'm surprised that this works with:
(but then again bash is always surprising)