Fix --disable-threading build

Fix --disable-threading build

PR build/29110 points out that GDB fails to build on mingw when the
"win32" thread model is in use.  It turns out that the Fedora cross
tools using the "posix" thread model, which somehow manages to support
std::future, whereas the win32 model does not.

While looking into this, I found that the configuring with
--disable-threading will also cause a build failure.

This patch fixes this build by introducing a compatibility wrapper for
std::future.

I am not able to test the win32 thread model build, but I'm going to
ask the reporter to try this patch.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29110