Support targeting Windows XP

[larsbergstrom]

It would be great for Rust to be able to build code that targets Windows down to Windows XP SP2. There are still a significant number of Firefox users on that platform, so any Rust code built into Firefox would also need to run on that platform.

The only issue I am aware of preventing this today is the use of condvars in std.

cc @brson

[alexcrichton]

This is a dupe of #12842, and I believe @vadimcn's list of APIs is mostly still applicable today for things we're using that are not on XP.

[alexcrichton]

Er as clarified with @larsbergstrom on IRC it possible to interpret #12842 as having the compiler run on XP and this as targeting XP, which are separate issues.

[vadimcn]

AFAIK, it was always understood that the compiler itself will never run on XP.

[larsbergstrom]

@vadimcn That other issue, #12842, has mixed comments and partial results evaluating both whether the toolchain runs on XP (which I do not care about) and whether binaries produces by rustc can run on XP (which I do care about). As long as the two are tracked separately - since I also suspect that running rustc.exe on XP is a non-goal - whatever selection of issues is fine by me.