[RS6000] PC-relative TLS support

[RS6000] PC-relative TLS support

Supporting TLS for -mpcrel turns out to be relatively simple.  The
existing TLSGD and TLSLD unspecs happily can have their GOT pointer
reg element replaced with zero, refelecting the fact that optimisation
of calls to __tls_get_addr when pc-rel won't use the GOT pointer.
Some other insns also can be reused, and just a few added.

	* config/rs6000/predicates.md (unspec_tls): Allow const0_rtx for got
	element of unspec vec.
	* config/rs6000/rs6000.c (rs6000_legitimize_tls_address): Support
	PC-relative TLS.
	* config/rs6000/rs6000.md (UNSPEC_TLSTLS_PCREL): New unspec.
	(tls_gd_pcrel, tls_ld_pcrel): New insns.
	(tls_dtprel, tls_tprel): Set attr prefixed when tls_size is not 16.
	(tls_got_tprel_pcrel, tls_tls_pcrel): New insns.

From-SVN: r278076