Emit the TLS initialization function pointers into the correct section.

The `llvm.tls_init_funcs' (created by the front-end) holds pointers to the TLS
initialization functions. These need to be placed into the correct section so
that they are run before `main()'.

<rdar://problem/13733006>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180737 91177308-0d34-0410-b5e6-96231b3b80d8
Improve documentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180738 91177308-0d34-0410-b5e6-96231b3b80d8