[ELF] Set up initial live symbol(s) to avoid incorrect reclaim of atoms.

If no initial live symbols are set up, and deadStrip() == true,
the Resolver ends up reclaiming all the symbols that aren't absolute. This is wrong.
This patch fixes the issue by setting entrySymbolName() as live, and this allows
us to self-host lld when --gc-sections is enabled. There are still quite a few problems
with --gc-sections (test failures), so the option can't be enabled by default.

Differential Revision:	D7926
Reviewed by:	ruiu, shankarke

llvm-svn: 230737