NPTL uses SIGRTMIN (signal 32) to effect thread cancellation.
Go's runtime replaces NPTL's signal handler with its own, and
ends up aborting if a C library that ends up calling
pthread_cancel is used.

This patch prevents runtime from replacing NPTL's handler.

Fixes #6997.

R=golang-codereviews, iant, dvyukov
CC=golang-codereviews
https://golang.org/cl/47540043