[runtimes] Run backdeployment CI on Github hosted runners (#109984)

This removes the need for macOS nodes in Buildkite. It also moves to the
proper way of testing backdeployment, which is to actually run on the
target OS itself, instead of using packaged dylibs from previous OS
versions and trying to emulate backdeployment with DYLD_LIBRARY_PATH.

As a drive-by change, also fix a few back-deployment annotations that
were incorrect and add support for minor versions in the Lit feature
determining availability from the target triple.

NOKEYCHECK=True
GitOrigin-RevId: 0547e573c555445e37db5c3bc92ee72274e19b69

Author: ldionne

test/configs/apple-libunwind-backdeployment.cfg.in

@@ -0,0 +1,41 @@
+# Testing configuration for back-deployment against the system-provided libunwind.
+#
+# Under this configuration, we compile and link all the test suite against the just-built
+# libunwind, but we run against the system libunwind.
+
+import os, site
+site.addsitedir(os.path.join('@LIBUNWIND_LIBCXX_PATH@', 'utils'))
+import libcxx.test.params, libcxx.test.config, libcxx.test.dsl
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+config.substitutions.append(('%{flags}',
+    '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
+))
+config.substitutions.append(('%{compile_flags}',
+    '-nostdinc++ -I %{include}'
+))
+config.substitutions.append(('%{link_flags}',
+    '-nostdlib++ -L %{lib} -lc++ -lunwind'
+))
+config.substitutions.append(('%{exec}',
+    '%{executor} --execdir %T -- '
+))
+
+config.stdlib = 'apple-libc++'
+config.using_system_stdlib = True
+
+# TODO: This is a giant hack, but we need to change the install_name of libunwind.dylib because the
+#       upstream configuration can't currently produce a libunwind.dylib that is compatible with the
+#       Apple system one.
+import subprocess
+subprocess.check_call(['install_name_tool', '-id', '/usr/lib/system/libunwind.dylib', '@LIBUNWIND_TESTING_INSTALL_PREFIX@/lib/libunwind.dylib'])
+
+import os, site
+import libcxx.test.params, libcxx.test.config
+libcxx.test.config.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)