PNaCl ABI: Disallow the "available_externally" linkage type

"define available_externally void @foo()" would mean that the
definition of foo() can be dropped from the pexe, because a definition
of foo() can be found in an external native library.  This is not
something we support for a PNaCl pexe.

Clang generates "available_externally" for non-static inline function
definitions at -O1, but not at -O0, and generally not at -O2 either
because inlining removes the "available_externally" definition.

"available_externally" gets removed at bitcode link time, so we can
disallow it in the ABI checker without any further work required.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3495
TEST=*.ll tests + PNaCl toolchain trybots

Review URL: https://codereview.chromium.org/17035015

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