Really use -mtls-use-call in libstdc++ build.
Really use -mtls-use-call in libstdc++ build.

The change in http://codereview.chromium.org/6825080 had a missing
backslash that made it omit the CXXFLAGS_FOR_TARGET setting and thus
defeat its purpose.  This escaped testing in the mayhem of changes.

I've also added a comment.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1595
TEST=trybot nacl-toolchain-lucid64-glibc

Review URL: http://codereview.chromium.org/6833003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@4848 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client