[RuntimeDyld] Ignore ST_FILE symbols when constructing GlobalSymbolTable

Summary: ELF's STT_File symbols may overlap with regular globals in
other files, so we should ignore them here in order to avoid having
bogus entries in the symbol table that confuse us when resolving relocations.

Reviewers: lhames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13888

llvm-svn: 250942