[AVR] Add the AVR builder 'llvm-avr-linux' back, pulling from GitHub

[dylanmckay]

Summary

This patch brings back the existing AVR builder that was removed in
3b645861c5110a9d11bf5f6d936ca074212cc452 due to it being incompatible
with the GitHub only repo we now have.

The builder now uses the 'getClangCMakeBuildFactory()' factory, instead
of 'getLLVMCMakeBuildFactory()' as it used to.

[dylanmckay]

Gentle ping

[gkistanova]

LGTM

[dylanmckay]

Landed in d94f2381197b14fa3e65eafd17ad4ceaf972ae34.

buildbot/osuosl/master/config/builders.py

return [
                           "-DLLVM_TARGETS_TO_BUILD='ARM;AArch64;X86'",
                           #"-DCMAKE_C_FLAGS=''",
                           #"-DCMAKE_CXX_FLAGS=''",
                           ],
                       env={'BUCKET': 'llvm-build-artifacts'}
                ),
          'category': 'clang'},
 
+        {'name' : "llvm-avr-linux",
+         'slavenames' : ["avr-build-01"],
+         'builddir' : "llvm-avr-linux",
+         'factory': ClangBuilder.getClangCMakeBuildFactory(
+                      jobs=8,
+                      clean=False,
+                      checkout_lld=False,
+                      extra_cmake_args=['-DLLVM_ENABLE_ASSERTIONS=ON',
+                                        # We need to compile the X86 backend due to a few generic CodeGen tests.
+                                        '-DLLVM_TARGETS_TO_BUILD=X86',
+                                        '-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR',
+                                        '-DBUILD_SHARED_LIBS=ON'])},
+
         {'name' : "clang-solaris11-amd64",
          'slavenames' : ["solaris11-amd64"],
          'builddir' : "clang-solaris11-amd64",
          'factory': ClangBuilder.getClangCMakeBuildFactory(
                       jobs=8,
                       clean=False,
                       checkout_lld=False,
                       extra_cmake_args=['-DLLVM_ENABLE_ASSERTIONS=ON',