[PECOFF] Support TLS callbacks.

The contents from section .CRT$XLA to .CRT$XLZ is an array of function
pointers. They are called by the runtime when a new thread is created
or (gracefully) terminated.

You can make your own initialization function to be called by that
mechanism. All you have to do is:

- Define a pointer to a function in a .CRT$XL* section using pragma
- Make an external reference to "__tls_used" symbol

That technique is used in many projects. This patch is to support that.

What this patch does is to set the relative virtual address of
"__tls_used" to the PECOFF directory table. __tls_used is actually a
struct containing pointers to a symbol in .CRT$XLA and another symbol
in .CRT$XLZ. The runtime looks at the directory table, gets the address
of the struct, and call the function pointers between XLA and XLZ.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@218007 91177308-0d34-0410-b5e6-96231b3b80d8