amdgcn: Support XNACK mode

amdgcn: Support XNACK mode

The XNACK feature allows memory load instructions to restart safely following
a page-miss interrupt.  This is useful for shared-memory devices, like APUs,
and to implement OpenMP Unified Shared Memory.

To support the feature we must be able to set the appropriate meta-data and
set the load instructions to early-clobber.  When the port supports scheduling
of s_waitcnt instructions there will be further requirements.

gcc/ChangeLog:

	* config/gcn/gcn-hsa.h (NO_XNACK): Ignore missing -march.
	(XNACKOPT): Match on/off; ignore any.
	* config/gcn/gcn-valu.md (gather<mode>_insn_1offset<exec>):
	Add xnack compatible alternatives.
	(gather<mode>_insn_2offsets<exec>): Likewise.
	* config/gcn/gcn.cc (gcn_option_override): Permit -mxnack for devices
	other than Fiji and gfx1030.
	(gcn_expand_epilogue): Remove early-clobber problems.
	(gcn_hsa_declare_function_name): Obey -mxnack setting.
	* config/gcn/gcn.md (xnack): New attribute.
	(enabled): Rework to include "xnack" attribute.
	(*movbi): Add xnack compatible alternatives.
	(*mov<mode>_insn): Likewise.
	(*mov<mode>_insn): Likewise.
	(*mov<mode>_insn): Likewise.
	(*movti_insn): Likewise.
	* config/gcn/gcn.opt (-mxnack): Change the default to "any".
	* doc/invoke.texi: Remove placeholder notice for -mxnack.