syscall: don't define PtraceRegs for Alpha
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend