Check for 64-bit phdrs for the TLS template in any x86-64 compilation
Check for 64-bit phdrs for the TLS template in any x86-64 compilation

This allows PNaCl to be used in x86-64-biased mode (when __pnacl__ is not predefined by the frontend) for building the IRT.
gcc's unused function result warning is stricter than clang's: it complained about the previous construction, so  use the results of the read_phdr calls to determine if the headers were found, rather than checking whether the info was filled in.

R=mseaborn@chromium.org
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3577

Review URL: https://codereview.chromium.org/20553006

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11865 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client