xtask: Add support for building watchOS targets.

[pixlwave]

This PR makes the following changes to the Swift xtask:

• Add support for building Tier 3 targets (by using nightly and -Zbuild-std).
• Add the watchos targets (I ignored armv7k-apple-watchos as the last supported device with that was the Series 3 which is became unsupported as of watchOS 9).
• Add a CLI flag to include the tier 3 targets when building all targets (which are ignored by default).
• Add a linker workaround for watchOS simulator builds.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.61%. Comparing base (9ab886f) to head (17495e7).
⚠️ Report is 29 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5872      +/-   ##
==========================================
+ Coverage   88.59%   88.61%   +0.01%     
==========================================
  Files         363      363              
  Lines      102888   102888              
  Branches   102888   102888              
==========================================
+ Hits        91153    91171      +18     
+ Misses       7496     7477      -19     
- Partials     4239     4240       +1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5872 will not alter performance

_{Comparing doug/watchos (17495e7) with main (5d396e4)}

Summary

✅ 50 untouched

[poljar]

Alright, can't spot anything wrong with it and I trust you didn't break your own build 😅