WebAssembly: remove 'external' from test

Summary: Linker testing was sad at seeing an unresolved external symbol. For now don't do that: it's valid but we're not playing with multi-file linking yet, and the LLVM tests are used as hacky sanity tests for single-file linking (the GCC torture tests are much better for this purpose). Another solution would be to use '.extern' to make the intent explicit (don't simple-file link this, there's an unresolved symbol), some assemblers use '.extern' while others ignore it, so we wouldn't really be inventing anything new.

Reviewers: sunfish, kripken

Subscribers: jfb, llvm-commits, dschuff

Differential Revision: http://reviews.llvm.org/D15753

llvm-svn: 256353