syscall: don't use pt_regs in clone_linux.c
syscall: don't use pt_regs in clone_linux.c

It's unnecessary and it reportedly breaks the build on arm64 GNU/Linux.

Change-Id: I39859d4e5747428586b5119fda972f9903299fec
Reviewed-on: https://go-review.googlesource.com/30978
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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