Allow undefined references to TLS symbols.

Allow undefined references to TLS symbols.

When --warn-unresolved-symbols is used, gold tries to create a dynamic relocation
for it, and gives an internal error if the TLS segment has not already been
created. This patch allows the IE-to-LE optimization for an undefined symbol
when building an executable, which suppresses the dynamic relocation, and
relaxes the requirement to have a TLS segment when applying a relocation for
an undefined symbol.

2015-01-28  Cary Coutant  <ccoutant@google.com>

gold/
	* x86_64.cc (Target_x86_64::Scan::global): Allow IE-to-LE optimization
	for undef TLS symbols.
	(Target_x86_64::Relocate::relocate_tls): Likewise.
	(Target_x86_64::Relocate::tls_ie_to_le): Likewise.