SWIG generates cgo input files in the work directory. When those files
are passed directly to cgo, cgo generates long file names that include
the object directory (with slashes replaced by underscores). Instead,
use cgo's new -srcdir option to give it short file names.

When using both SWIG and cgo, copy the cgo files into the object
directory first.

Use a shorter object file name when compiling the C file generated by
SWIG.

Fixes #17070.

Change-Id: Ic558603f1731636d9999f3130ad0224b24bd7dcb
Reviewed-on: https://go-review.googlesource.com/32485
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>