The change to signal_amd64.c from CL 15790043 was not merged correctly.

This CL reapplies the change, renaming the file to signal_amd64x.c and adds the appropriate build tags.

LGTM=iant, bradfitz
R=rsc, iant, bradfitz
CC=golang-codereviews
https://golang.org/cl/72790043