Simplify SwiftPM builds

[rauhul]

Two important embedded PRs have been merged into SwiftPM main that should allow us to drop some compiler and swift build flags:
Do not try to add rpath arguments on noneOS
Use mergeable symbols in embedded

Adopt the changes when a nightly is available.

• remove brew-install llvm-toolchain from rpi-4/5
• remove --disable-local-rpath
• remove -allow_dead_duplicates