Force __pnacl_pso_root to be an external declaration.

Force __pnacl_pso_root to be an external declaration.

This CL updates "isPNaClABIExternalName" -- Subzero checked for the symbol
"__pnacl_pso_root" as a function, but it is a declaration, and should be
checked as one. Additionally, when the PNaClTranslator is verifying the linkage
of declarations, allow "__pnacl_pso_root" to be flipped to external as a special
case.

TEST=pnacl-translate -pso --use-sz -arch x86-32-nonsfi test_pll.final.pso
BUG=https://bugs.chromium.org/p/nativeclient/issues/detail?id=4351
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1740033002 .