[COFF] Don't hard-code the load configuration size

[COFF] Don't hard-code the load configuration size

The load configuration directory is a structure whose size varies as the
OS gains additional functionality.  To account for this, the structure's
layout begins with a size field; this allows loaders to know which
fields are available.

However, LLD hard-coded the sizes (112 bytes for 64-bit and 64 for
32-bit).  This means that we might not inform the loader of all the
pertinent fields or we might claim that there are more fields than are
actually present.

To correctly account for this, the size field must be loaded from the
_load_config_used symbol.

N.B.  The COFF spec is either wrong or out of date, the load
configuration directory is not correctly documented in the
specification: it omits the size field.

llvm-svn: 263543