Support AARCH64_TLSLD_ADD_DTPREL_* relocations.

Support AARCH64_TLSLD_ADD_DTPREL_* relocations.

Also Change _TLS_MODULE_BASE_. Always let it point to the start
of TLS segment.

2015-03-28  Jing Yu  <jingyu@google.com>

	* aarch64-reloc.def: New TLSLD_ADD_DTPREL_HI12,
	TLSLD_ADD_DTPREL_LO12_NC.
	* aarch64.cc (Target_aarch64::define_tls_base_symbol): Always
	let _TLS_MODULE_BASE_ point to the start of tls segment.
	(Target_aarch64::optimize_tls_reloc): Add cases for
	R_AARCH64_TLSLD_ADD_DTPREL_HI12 and
	R_AARCH64_TLSLD_ADD_DTPREL_LO12_NC.
	(Target_aarch64::Scan::local): Likewise.
	(Target_aarch64::Scan::global): Likewise.
	(Target_aarch64::Relocate::relocate): Likewise.
	(Target_aarch64::Relocate::relocate_tls): Likewise. And remove
	subtracting tls segment size from symbol value for
	TLSLD_*_DTPREL relocations.