Add support for uwp Windows targets

[Susurrus]

Problem
Installing any uwp-windows target fails.

Steps:

$ rustup target install x86_64-uwp-windows-gnu
error: toolchain 'stable-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'x86_64-uwp-windows-gnu'; did you mean 'x86_64-pc-windows-gnu'?

Possible Solution(s)
Add support for uwp targets

Notes
The targets aarch64-uwp-windows-msvc, i686-uwp-windows-gnu, i686-uwp-windows-msvc, x86_64-uwp-windows-gnu, and x86_64-uwp-windows-msvc are supported in Rust as of release 1.38.0 [see https://github.com/rust-lang/rust/pull/60260]/

Output of rustup --version: rustup 1.21.1 (7832b2e 2019-12-20)
Output of rustup show: Default host: x86_64-unknown-linux-gnu
rustup home: /home/susurrus/.rustup

installed toolchains

stable-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu
1.36.0-x86_64-unknown-linux-gnu

installed targets for active toolchain

x86_64-apple-darwin
x86_64-pc-windows-gnu
x86_64-pc-windows-msvc
x86_64-unknown-linux-gnu
x86_64-unknown-linux-musl
x86_64-unknown-redox

active toolchain

stable-x86_64-unknown-linux-gnu (default)
rustc 1.44.0 (49cae5576 2020-06-01)

[kinnison]

The uwp targets are tier 3 and are not included in release channels - see https://forge.rust-lang.org/release/platform-support.html

At this time there's nothing for rustup to do.