Adopt the new runtime layout in the Swift compiler for Linux (and possibly *BSD)

[edymtt]

Description

When building compiler-rt with LLVM_ENABLE_RUNTIMES under Linux the underlying build system assumes we are using the new runtime layout -- as a result, the Swift compiler is not able to find any of the builtins and sanitizer libraries it needs to work.

For now in #80174 I am reverting back to the old layout, but long term we want the Swift compiler to work with the new layout under Linux (and in general any configuration that builds LLVM with LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)

Additional information

No response