Add a centos7 build of LLVM and switch linux build to it

[ChrisCummins]

No description provided.

[ChrisCummins]

On a Pop! OS 20.10 machine I'm seeing load-time errors:

$ bazel test //...
INFO: Analyzed 112 targets (5 packages loaded, 3698 targets configured).
INFO: Found 108 targets and 4 test targets...
FAIL: //examples:clang-compile-10.0.0 (see /mnt/cache/cec/bazel/_bazel_cec/3502c1386739d8b5c8fef37b572870b1/execroot/llvm/bazel-out/k8-fastbuild/testlogs/examples/clang-compile-10.0.0/test.log)
ERROR: /home/cec/src/bazel_llvm/examples/BUILD:12:10: Linking of rule '//examples:clang-interpreter' failed (Exit 1): gcc failed: error executing command /bin/gcc @bazel-out/k8-fastbuild/bin/examples/clang-interpreter-2.params

Use --sandbox_debug to see verbose messages from the sandbox gcc failed: error executing command /bin/gcc @bazel-out/k8-fastbuild/bin/examples/clang-interpreter-2.params

Use --sandbox_debug to see verbose messages from the sandbox
bazel-out/k8-fastbuild/bin/examples/_objs/clang-interpreter/clang-interpreter.pic.o:clang-interpreter.cc:function GetExecutablePath[abi:cxx11](char const*, void*): error: undefined reference to 'llvm::sys::fs::getMainExecutable[abi:cxx11](char const*, void*)'
bazel-out/k8-fastbuild/bin/examples/_objs/clang-interpreter/clang-interpreter.pic.o:clang-interpreter.cc:function main: error: undefined reference to 'llvm::sys::getProcessTriple[abi:cxx11]()'
bazel-out/k8-fastbuild/bin/examples/_objs/clang-interpreter/clang-interpreter.pic.o:clang-interpreter.cc:function main: error: undefined reference to 'clang::CompilerInvocation::GetResourcesPath[abi:cxx11](char const*, void*)'
bazel-out/k8-fastbuild/bin/examples/_objs/clang-interpreter/clang-interpreter.pic.o:clang-interpreter.cc:function llvm::sys::DynamicLibrary::LoadLibraryPermanently(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*): error: undefined reference to 'llvm::sys::DynamicLibrary::getPermanentLibrary(char const*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*)'
bazel-out/k8-fastbuild/bin/examples/_objs/clang-interpreter/clang-interpreter.pic.o:clang-interpreter.cc:function llvm::orc::SimpleJIT::SimpleJIT(std::unique_ptr<llvm::TargetMachine, std::default_delete<llvm::TargetMachine> >, llvm::DataLayout, std::unique_ptr<llvm::orc::DynamicLibrarySearchGenerator, std::default_delete<llvm::orc::DynamicLibrarySearchGenerator> >): error: undefined reference to 'llvm::orc::ExecutionSession::createJITDylib(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)'
collect2: error: ld returned 1 exit status
INFO: Elapsed time: 112.454s, Critical Path: 6.58s
INFO: 228 processes: 215 internal, 13 linux-sandbox.
FAILED: Build did NOT complete successfully
//examples:clang-interpreter-test                                     NO STATUS
//examples:fibonacci_10.0.0                                              PASSED in 0.2s
//examples:hello-world-10.0.0                                            PASSED in 0.0s
//examples:clang-compile-10.0.0                                          FAILED in 0.0s
  /mnt/cache/cec/bazel/_bazel_cec/3502c1386739d8b5c8fef37b572870b1/execroot/llvm/bazel-out/k8-fastbuild/testlogs/examples/clang-compile-10.0.0/test.log

Executed 3 out of 4 tests: 2 tests pass, 1 fails locally and 1 was skipped.
There were tests whose specified size is too big. Use the --test_verbose_timeout_warnings command line optio
FAILED: Build did NOT complete successfully