LLVM_ENABLE_RUNTIMES=flang-rt for UnifiedTreeBuilder builders

[Meinersbur]

Add depends_on_projects=['flang-rt'] and checks=['check-flang-rt'] to Linaro's builders that are based on UnifiedTreeBuilder. This prepares the removal of the "projects" build of the flang runtime in llvm/llvm-project#124126.

Split off from #333

Affected builders:

• flang-aarch64-dylib
• flang-aarch64-sharedlibs
• flang-aarch64-debug-reverse-iteration
• flang-aarch64-libcxx
• flang-aarch64-release
• flang-aarch64-rel-assert
• flang-aarch64-latest-gcc

Affected workers:

• linaro-flang-aarch64-dylib
• linaro-flang-aarch64-sharedlibs
• linaro-flang-aarch64-debug-reverse-iteration
• linaro-flang-aarch64-libcxx
• linaro-flang-aarch64-release
• linaro-flang-aarch64-rel-assert
• linaro-flang-aarch64-latest-gcc

I could locally confirm that flang-aarch64-dylib works on a x86_64 host (since I don't have an aarch machine other than a Raspberry Pi readily available)

[luporl]

LGTM.
@ceseo, you've already confirmed that LLVM_ENABLE_RUNTIMES=flang-rt and check-flang-rt work on AArch64/Linux, right?

[ceseo]

LGTM.

@ceseo, you've already confirmed that LLVM_ENABLE_RUNTIMES=flang-rt and check-flang-rt work on AArch64/Linux, right?

Yes, both stage1 and stage2.