Add 'errno.h' to the Darwin SDK.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@185104 91177308-0d34-0410-b5e6-96231b3b80d8
Declare 'strerror' so that we can use it with errno.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@185105 91177308-0d34-0410-b5e6-96231b3b80d8
Revert hack that omits errno on Darwin platforms. We now have an acceptable 'errno' header.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@185106 91177308-0d34-0410-b5e6-96231b3b80d8