[libc++] Add a from-scratch testing config for GCC

libcxx/test/configs/llvm-libc++-shared-gcc.cfg.in

+# This testing configuration handles running the test suite against LLVM's libc++
+# using a shared library, with GCC. This is done differently from Clang because
+# GCC does not support the -nostdlib++ command-line flag.
+
+import sys
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+config.substitutions.append(('%{flags}', ''))
+config.substitutions.append(('%{compile_flags}',
+    '-nostdinc++ -isystem %{include} -isystem %{target-include} -I %{libcxx}/test/support'
+))
+config.substitutions.append(('%{link_flags}',
+    '-L %{lib} -Wl,-rpath,%{lib} -nodefaultlibs -lc++ -lm -lgcc_s -lgcc -lpthread -lc -lgcc_s -lgcc -latomic'
+))
+config.substitutions.append(('%{exec}',
+    '{} %{{libcxx}}/utils/run.py --execdir %T -- '.format(sys.executable)
+))
+
+import os, site
+site.addsitedir(os.path.join('@LIBCXX_SOURCE_DIR@', 'utils'))
+import libcxx.test.params, libcxx.test.newconfig, libcxx.test.newconfig
+libcxx.test.newconfig.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)

libcxx/utils/ci/run-buildbot

generic-clang-12)
     clean
     generate-cmake -DLIBCXX_TEST_CONFIG="llvm-libc++-shared.cfg.in"
     check-runtimes
 ;;
 generic-gcc)
     export CC=gcc-11
     export CXX=g++-11
     clean
-    generate-cmake
+    generate-cmake -DLIBCXX_TEST_CONFIG="llvm-libc++-shared-gcc.cfg.in"
     check-runtimes
 ;;
 generic-gcc-cxx11)
     export CC=gcc-11
     export CXX=g++-11
     clean
-    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-cxx11.cmake"
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-cxx11.cmake" \
+                   -DLIBCXX_TEST_CONFIG="llvm-libc++-shared-gcc.cfg.in"
     check-runtimes
 ;;
 generic-asan)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-asan.cmake" \
                    -DLIBCXX_TEST_CONFIG="llvm-libc++-shared.cfg.in"
     check-runtimes
 ;;