Put __pnacl_real_irt_interface variable in bss instead of common.

Then we don't have to handle SHN_COMMON in the native link.
Normally, pnacl-clang adds -fno-common to force definition
of common symbols. However, the x86 shims are compiled by
nacl-gcc so it showed up.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=3762
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/116043012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243473 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243473