Add Windows XP target support

[briansmith]

[It's not clear that any ring users actually need XP support. Unless/until it is clear that it is needed, we should avoid adding it. I'm filing this issue here to keep track of what I know about what would be required for XP support.]

Rust only supports Vista and later in "tier 1." However, Firefox and maybe some other Rust applications do target Windows XP, and there are some hacks in libstd to make this partially work.

ring uses lazy_static and it's likely that lazy_static doesn't work correctly on XP because XP probably doesn't have the necessary concurrency primitives.

Also, we'd need to build the C & C++ code using the v140_xp toolchain.

I filed rust-lang/rust#37638 to get the Rust toolchain to support optionally enabling XP support in a consistent fashion throughout the ecosystem. If we implement this in the meantime, we could add a "windows-xp" feature to ring that is off-by-default, and which can be enabled by the application if they want to target XP too.

The hard part of this project is getting test coverage. We'd need to do it in a VM and can't use AppVeyor, since it doesn't support XP.

[briansmith]

See, at least:

• Panic broken on Windows XP rust-lang/rust#34538 (comment)
• Investigate fallback implementations on Windows XP rust-lang/rust#26654 (comment)

[briansmith]

Closing this since nobody seems to want it.