syscall: don't define PtraceRegs for Alpha

It's now defined by mksysinfo.sh.

Patch by Uros Bizjak.

Change-Id: Ic728fef84ae7e0077176a1a73f9c74558420364e
Reviewed-on: https://go-review.googlesource.com/46712
Reviewed-by: Ian Lance Taylor <iant@golang.org>