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/17451Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testcshared/main2.c

@@ -9,7 +9,7 @@
 #include <time.h>
 #include <unistd.h>
 
-#define fd (10)
+#define fd (100)
 
 // Tests libgo2.so, which does not export any functions.
 // Read a string from the file descriptor and print it.

misc/cgo/testcshared/src/libgo2/libgo2.go

@@ -21,7 +21,7 @@ import (
 // that the C code can also use.
 
 const (
-	fd = 10
+	fd = 100
 )
 
 func init() {