misc/cgo/testcshared: use fd 100 instead of fd 10 for back-channel communication

misc/cgo/testcshared: use fd 100 instead of fd 10 for back-channel communication

There is a report that fd 10 is already in use when run on some OS X machines.
I don't see how, and I can't reproduce the problem on my own OS X machine,
but it's easy enough to fix.

Fixes #12161.

Change-Id: I73511bdd91258ecda181d60d2829add746d1198b
Reviewed-on: https://go-review.googlesource.com/17451
Reviewed-by: Ian Lance Taylor <iant@golang.org>