[PECOFF] Emit x64 delay-import wrapper function

Previously we supported x86 only. This patch is to support x64.

The array of pointers to delay-loaded functions points the
DLL delay loading function at start-up. When a function is called
for the first time, the delay loading function gets called and
then rewrite the function pointer array.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220096 91177308-0d34-0410-b5e6-96231b3b80d8