[WebAssembly] Fix symbol type for addresses of external functions

These symbols were previously not being marked as functions
so were appearing as globals instead, and with the incorrect
relocation type.

Without this fix, objects that take address of external
functions include them as global imports rather than function
imports which then fails at link time.

Differential Revision: https://reviews.llvm.org/D34068

llvm-svn: 305263