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 (XNACKOPT): New macro.
	(ASM_SPEC): Use XNACKOPT.
	* config/gcn/gcn-opts.h (enum sram_ecc_type): Rename to ...
	(enum hsaco_attr_type): ... this, and generalize the names.
	(TARGET_XNACK): New macro.
	* config/gcn/gcn-valu.md (gather<mode>_insn_1offset<exec>):
	Add xnack compatible alternatives.
	(gather<mode>_insn_2offsets<exec>): Likewise.
	* config/gcn/gcn.c (gcn_option_override): Permit -mxnack for devices
	other than Fiji.
	(gcn_expand_epilogue): Remove early-clobber problems.
	(output_file_start): Emit xnack attributes.
	(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): Add the "on/off/any" syntax.
	(sram_ecc_type): Rename to ...
	(hsaco_attr_type: ... this.)
	* config/gcn/mkoffload.c (SET_XNACK_ANY): New macro.
	(TEST_XNACK): Delete.
	(TEST_XNACK_ANY): New macro.
	(TEST_XNACK_ON): New macro.
	(main): Support the new -mxnack=on/off/any syntax.