Write to an in-memory buffer before writing to an actual output file

We don't want to leave a partial assembly output if the compiler
fails during compilation.

Technically speaking, there's still a risk of leaving a partially-
written output file if the compiler dies during file copy. To fix
that, we have to write to a temporary file in the same filesystem as
the final output file and rename the temporary file to atomically
replace the output file. We don't do that in this patch for the sake
of succinctness, though.