[WebAssembly] Only treat imports/exports as symbols when reading relocatable object files

This change only treats imported and exports functions and globals
as symbol table entries the object has a "linking" section (i.e. it is
relocatable object file).

In this case all globals must be of type I32 and initialized with
i32.const.  This was previously being assumed but not checked for and
was causing a failure on big endian machines due to using the wrong
value of then union.

See: https://bugs.llvm.org/show_bug.cgi?id=34487

Differential Revision: https://reviews.llvm.org/D37497

llvm-svn: 312674