[runtimes] Simplify testing configurations on GCC by using -nostdlib++ (llvm#67021)

Since GCC now supports -nostdlib++, we can remove some complexity in the
test configurations and do the same as Clang. However, we can't fully
remove the GCC test configuration for libc++ because we apparently need
to explicitly link against libm for some tests to work.

Author: ldionne

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

@@ -1,6 +1,6 @@
 # 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.
+# we need to explicitly link against libm for some tests with GCC.
 
 lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
 
@@ -9,7 +9,7 @@ config.substitutions.append(('%{compile_flags}',
     '-nostdinc++ -I %{include} -I %{target-include} -I %{libcxx}/test/support'
 ))
 config.substitutions.append(('%{link_flags}',
-    '-L %{lib} -Wl,-rpath,%{lib} -nodefaultlibs -lc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc -latomic'
+    '-nostdlib++ -L %{lib} -Wl,-rpath,%{lib} -lc++ -lm'
 ))
 config.substitutions.append(('%{exec}',
     '%{executor} --execdir %T -- '

libcxxabi/CMakeLists.txt

@@ -144,9 +144,7 @@ endif()
 option(LIBCXXABI_HERMETIC_STATIC_LIBRARY
   "Do not export any symbols from the static library." ${LIBCXXABI_HERMETIC_STATIC_LIBRARY_DEFAULT})
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-  set(LIBCXXABI_DEFAULT_TEST_CONFIG "llvm-libc++abi-shared-gcc.cfg.in")
-elseif(MINGW)
+if(MINGW)
   set(LIBCXXABI_DEFAULT_TEST_CONFIG "llvm-libc++abi-mingw.cfg.in")
 elseif(WIN32) # clang-cl
   if (LIBCXXABI_ENABLE_SHARED)