libgo: redefine ia64 struct names around linux/ptrace.h

Avoid https://sourceware.org/bugzilla/show_bug.cgi?id=762.

Patch by Andreas Schwab.

Change-Id: I4b2f0bc3a403020f7d5a230f7defbf8ad380cfb8
Reviewed-on: https://go-review.googlesource.com/46711
Reviewed-by: Ian Lance Taylor <iant@golang.org>