Apple: Refactor deployment target version parsing

[madsmtm]

Refactor deployment target parsing to make it easier to do #129342 (I wanted to make sure of all the places that std::env::var is called).

Specifically, my goal was to minimize the amount of target-specific configuration, so to that end I renamed the opts function that generates the TargetOptions to base, and made it return the LLVM target and target_arch too. In the future, I would like to move even more out of the target files and into spec::apple, as it makes it easier for me to maintain.

For example, this fixed a bug in aarch64-apple-watchos, which wasn't passing the deployment target as part of the LLVM triple. This (probably) fixes #123582 and fixes #107630.

We also now parse the patch version of deployment targets, allowing the user to specify e.g. MACOSX_DEPLOYMENT_TARGET=10.12.6.

Finally, this fixes the LLVM target name for visionOS, it should be *-apple-xros and not *-apple-visionos.

Since I have changed all the Apple targets here, I smoke-tested my changes by running the following:

# Build each target
./x build library --target="aarch64-apple-darwin,aarch64-apple-ios,aarch64-apple-ios-macabi,aarch64-apple-ios-sim,aarch64-apple-tvos,aarch64-apple-tvos-sim,aarch64-apple-visionos,aarch64-apple-visionos-sim,aarch64-apple-watchos,aarch64-apple-watchos-sim,arm64_32-apple-watchos,arm64e-apple-ios,armv7k-apple-watchos,armv7s-apple-ios,i386-apple-ios,x86_64-apple-darwin,x86_64-apple-ios,x86_64-apple-ios-macabi,x86_64-apple-tvos,x86_64-apple-watchos-sim,x86_64h-apple-darwin"

# Test that we can still at least link basic projects
cargo new foobar && cd foobar && cargo +stage1 build --target=aarch64-apple-darwin --target=aarch64-apple-ios --target=aarch64-apple-ios-macabi --target=aarch64-apple-ios-sim --target=aarch64-apple-tvos --target=aarch64-apple-tvos-sim --target=aarch64-apple-visionos --target=aarch64-apple-visionos-sim --target=aarch64-apple-watchos --target=aarch64-apple-watchos-sim --target=arm64_32-apple-watchos --target=armv7s-apple-ios --target=i386-apple-ios --target=x86_64-apple-darwin --target=x86_64-apple-ios --target=x86_64-apple-ios-macabi --target=x86_64-apple-tvos --target=x86_64-apple-watchos-sim --target=x86_64h-apple-darwin

I couldn't build for the arm64e-apple-darwin target, the armv7k-apple-watchos and arm64e-apple-ios targets failed to link, and I know that the i686-apple-darwin target requires a bit of setup, but all of this is as it was before this PR.

r? thomcc

CC @BlackHoleFox

I would recommend using rollup=never when merging this, in case we need to bisect this later.