Move std_detect into stdlib

[Kobzol]

This PR moves the std_detect crate from stdarch to be a part of rust-lang/rust instead.

The first commit actually moves the whole directory from the stdarch Josh subtree, so that git blame history is kept intact. Then I had to make a few changes to appease tidy.

The most complex thing here is porting the tests. We can't have std_detect both in r-l/r and stdarch, because they could get desynchronized, so we have to perform the move more or less "atomically", which means that we also have to port all the existing std_detect tests from the stdarch repository.

The stdarch repo runs the following std_detect tests:

Build

The build-std-detect.sh script (https://github.com/rust-lang/stdarch/blob/e2b6512aed87df45294ae680181eeef7a802cd95/ci/build-std-detect.sh) builds std_detect using the nightly compiler for several targets. This will be subsumed by normal x build library on our Tier 1/2 targets. However, the stdarch repository also tests the following targets:

• aarch64-unknown-freebsd
• armv6-unknown-freebsd
• powerpc-unknown-freebsd
• powerpc64-unknown-freebsd
• aarch64-unknown-openbsd

Which we don't build/test on our CI currently. I think we have mostly two options here:

1. Ignore these targets
2. Create a special CI job that will build stage 1 rustc and then cross-compile std (or just the std_detect crate?) for these targets.

Documentation

The dox.sh script (https://github.com/rust-lang/stdarch/blob/3fec5adcd52a815f227805d4959a25b6402c7fd5/ci/dox.sh) builds and documents std_detect for several targets. All of them are Tier 2/we have dist- jobs for them, so I think that we can just skip this and let our normal CI subsume it?

Tests

The run.sh script (https://github.com/rust-lang/stdarch/blob/1b201cec2cca7465602a65ed6ae60517224b15f3/ci/run.sh) runs cargo test on std_detect with a bunch of variations of feature flags. This will be subsumed by x test library in our CI. The only problem is that stdarch runs these tests for a ludicrous number of targets:

        - tuple: i686-unknown-linux-gnu
        - tuple: x86_64-unknown-linux-gnu
        - tuple: arm-unknown-linux-gnueabihf
        - tuple: armv7-unknown-linux-gnueabihf
        - tuple: aarch64-unknown-linux-gnu
        - tuple: aarch64_be-unknown-linux-gnu
        - tuple: riscv32gc-unknown-linux-gnu
        - tuple: riscv64gc-unknown-linux-gnu
        - tuple: powerpc-unknown-linux-gnu
        - tuple: powerpc64-unknown-linux-gnu
        - tuple: powerpc64le-unknown-linux-gnu
        - tuple: s390x-unknown-linux-gnu
        - tuple: i586-unknown-linux-gnu
        - tuple: nvptx64-nvidia-cuda
        - tuple: thumbv6m-none-eabi
        - tuple: thumbv7m-none-eabi
        - tuple: thumbv7em-none-eabi
        - tuple: thumbv7em-none-eabihf
        - tuple: loongarch64-unknown-linux-gnu
        - tuple: wasm32-wasip1
        - tuple: x86_64-apple-darwin
        - tuple: x86_64-apple-ios-macabi
        - tuple: aarch64-apple-darwin
        - tuple: aarch64-apple-ios-macabi
        - tuple: x86_64-pc-windows-msvc
        - tuple: i686-pc-windows-msvc
        - tuple: aarch64-pc-windows-msvc
        - tuple: x86_64-pc-windows-gnu
        - tuple: aarch64-unknown-linux-gnu
        - tuple: aarch64_be-unknown-linux-gnu
        - tuple: armv7-unknown-linux-gnueabihf
        - tuple: loongarch64-unknown-linux-gnu
        - tuple: powerpc-unknown-linux-gnu
        - tuple: powerpc64-unknown-linux-gnu
        - tuple: powerpc64le-unknown-linux-gnu
        - tuple: riscv32gc-unknown-linux-gnu
        - tuple: riscv64gc-unknown-linux-gnu
        - tuple: s390x-unknown-linux-gnu
        - tuple: x86_64-unknown-linux-gnu
        - tuple: aarch64-apple-darwin
        - tuple: aarch64-apple-ios-macabi

We definitely do not run tests for all of these targets on our CI. So maybe include them into the special job that will cross-compile std_detect for Free/OpenBSD?

r? @ghost