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.

Previously, translating a pso with --use-sz caused the warning:
"cannot find entry symbol '__pnacl_pso_root'".
That warning is removed with this CL.

Fixes revert from https://codereview.chromium.org/1745783002/

TEST=external_declaration.ll
BUG=https://bugs.chromium.org/p/nativeclient/issues/detail?id=4351
R=kschimpf@google.com, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1774383002 .