[compiler-rt] Workaround using new Clang with an old NDK.

[danalbert]

Summary

We're using an old NDK and a new Clang. New Clangs default to
-stdlib=libc++ for Android, but those libraries cannot be found by
default with an old NDK. Use an explicit -stdlib=libstdc++ in the
cxx_mode_flags.

[eugenis]

Could you change config.clangxx in test/asan/lit.cfg instead?

[eugenis]

Or, rather, define something like common_cxxflags in test/lit.common.cfg and apply them in all sanitizer tests.
This script should not edit compilation flags.

[danalbert]

Thanks, hadn't found that file. Much cleaner.

[eugenis]

Yes, this is much better!
LGTM

[danalbert]

r346051

test/lit.common.cfg

 # If needed, add cflag for shadow scale.
 if config.asan_shadow_scale != '':
   config.target_cflags += " -mllvm -asan-mapping-scale=" + config.asan_shadow_scale
 
 # BFD linker in 64-bit android toolchains fails to find libm.so, which is a
 # transitive shared library dependency (via asan runtime).
 if config.android:
   config.target_cflags += " -pie -fuse-ld=gold -Wl,--enable-new-dtags"
+  config.cxx_mode_flags.append('-stdlib=libstdc++')
 
 # Clear some environment variables that might affect Clang.
 possibly_dangerous_env_vars = ['ASAN_OPTIONS', 'DFSAN_OPTIONS', 'LSAN_OPTIONS',
                                'MSAN_OPTIONS', 'UBSAN_OPTIONS',
                                'COMPILER_PATH', 'RC_DEBUG_OPTIONS',
                                'CINDEXTEST_PREAMBLE_FILE', 'LIBRARY_PATH',
                                'CPATH', 'C_INCLUDE_PATH', 'CPLUS_INCLUDE_PATH',
                                'OBJC_INCLUDE_PATH', 'OBJCPLUS_INCLUDE_PATH',