[embedded][Concurrency] Start building Concurrency.swiftmodule for armv6/armv6m

[kubamracek]

This is particularly relevant for the Pico (the MCU is RP2040, which is Cortex-M0+, which is ARMv6-M).

[kubamracek]

I think this will fail to build due to a clang crash, but I'll use this PR to track the progress on that.

[kubamracek]

@swift-ci please test

[kubamracek]

I think this is the relevant missing piece in LLVM: https://reviews.llvm.org/D49465
Looks like Thumb1 targets don't have tail cails support in LLVM, and Cortex-M0+ is Thumb1...

[kubamracek]

@swift-ci please test

[kubamracek]

FAILED: stdlib/public/Concurrency/CMakeFiles/embedded-concurrency-armv6m-apple-none-macho.dir/AsyncLet.cpp.o 
/usr/local/bin/sccache /Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/build/buildbot_incremental/llvm-macosx-x86_64/./bin/clang++ ... -c /Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/swift/stdlib/public/Concurrency/AsyncLet.cpp
fatal error: error in backend: failed to perform tail call elimination on a call site marked musttail
...
1.	<eof> parser at end of file
2.	Code generation
3.	Running pass 'Function Pass Manager' on module '/Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/swift/stdlib/public/Concurrency/AsyncLet.cpp'.
4.	Running pass 'ARM Instruction Selection' on function '@swift_asyncLet_get'
 #0 0x00000001093d4748 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int)
 #1 0x00000001093d2659 llvm::sys::RunSignalHandlers()
 #2 0x00000001093d3c91 llvm::sys::CleanupOnSignal(unsigned long)
 #3 0x000000010931f49e (anonymous namespace)::CrashRecoveryContextImpl::HandleCrash(int, unsigned long)
 #4 0x000000010931f423 llvm::CrashRecoveryContext::HandleExit(int)
 #5 0x00000001093cf24c llvm::sys::Process::Exit(int, bool)
 #6 0x000000010374a667 LLVMErrorHandler(void*, char const*, bool)
 #7 0x000000010932a1f8 llvm::report_fatal_error(llvm::Twine const&, bool)
 #8 0x000000010932a099 llvm::report_fatal_error(char const*, bool)
 #9 0x0000000103c3573b llvm::ARMTargetLowering::LowerCall(llvm::TargetLowering::CallLoweringInfo&, llvm::SmallVectorImpl<llvm::SDValue>&) const
#10 0x0000000104abe7ec llvm::TargetLowering::LowerCallTo(llvm::TargetLowering::CallLoweringInfo&) const
#11 0x0000000104adf952 llvm::SelectionDAGBuilder::lowerInvokable(llvm::TargetLowering::CallLoweringInfo&, llvm::BasicBlock const*)
#12 0x0000000104ac6eb3 llvm::SelectionDAGBuilder::LowerCallTo(llvm::CallBase const&, llvm::SDValue, bool, bool, llvm::BasicBlock const*, llvm::TargetLowering::PtrAuthInfo const*)
#13 0x0000000104aafb9a llvm::SelectionDAGBuilder::visitCall(llvm::CallInst const&)
#14 0x0000000104aa2db9 llvm::SelectionDAGBuilder::visit(llvm::Instruction const&)
...