Fix -emit-reloc against local symbols.

Previously, we drop symbols starting with .L from the symbol table, so
if there is a relocation that refers a .L symbol, it ended up
referencing a null -- which happened to be interpreted as an absolute
symbol.

This patch copies all symbols including local ones if -emit-reloc is
given.

Fixes https://bugs.llvm.org/show_bug.cgi?id=41385

Differential Revision: https://reviews.llvm.org/D60306

llvm-svn: 357885