Fixes a hang that can occur if a signal comes in during malloc calls.

Fixes a hang that can occur if a signal comes in during malloc calls.

We need to dereference the signals mutex during handler registration so that we force its construction. This is to prevent the first use being during handling an actual signal because you can't safely allocate memory in a signal handler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235914 91177308-0d34-0410-b5e6-96231b3b80d8