[LLD] [COFF] Avoid thread exhaustion on 32-bit Windows host

LLD on 32-bit Windows would frequently fail on large projects with
an exception "thread constructor failed: Exec format error".  The stack
trace pointed to this usage of std::async, and looking at the
implementation in libc++ it seems using std::async with
std::launch::async results in the immediate creation of a new thread
for every call.  This could result in a potentially unbounded number
of threads, depending on the number of input files.  This seems to
be hitting some limit in 32-bit Windows host.

I took the easy route, and only use threads on 64-bit Windows, not all
Windows as before.  I was thinking a more proper solution might
involve using a thread pool rather than blindly spawning any number
of new threads, but that may have other unforeseen consequences.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D105506