Check that rustc's output is writable before linking

[brson]

If the final output file is not writable we end up failing with a cryptic linker error and a dump of the gcc command line. This could largely be avoided by doing some check before running the linker.

[brson]

Maybe it's possible from the gcc error code to determine what the error was. That would potentially be less error prone than doing the check ahead of time.

[sammykim]

Even if the final output file is not writable, the error message is working properly.
When I make output file in /usr/bin, the error message is "Could not write output : error opening output file"

I am sure it's not still problem.

[catamorphism]

@sammykim is correct. Closing.