ARC: dw2 unwind: add infrastructure for adding cfi pseudo ops to asm

1. detect whether binutils supports the cfi pseudo ops
2. define conditional macros to generate the ops
3. define new ENTRY_CFI/END_CFI to annotate hand asm code.
   - Needed because we don't want to emit dwarf info in general ENTRY/END
     used by lowest level trap/exception/interrutp handlers as unwinder
     gets confused trying to unwind out of them. We want unwinder to
     instead stop when it hits onfo those routines
   - These provide minimal start/end cfi ops assuming routine doesn't
     touch stack memory/regs

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/dwarf.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2016-17 Synopsys, Inc. (www.synopsys.com)
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#ifndef _ASM_ARC_DWARF_H
+#define _ASM_ARC_DWARF_H
+
+#ifdef __ASSEMBLY__
+
+#ifdef ARC_DW2_UNWIND_AS_CFI
+
+#define CFI_STARTPROC	.cfi_startproc
+#define CFI_ENDPROC	.cfi_endproc
+#define CFI_DEF_CFA	.cfi_def_cfa
+#define CFI_REGISTER	.cfi_register
+#define CFI_REL_OFFSET	.cfi_rel_offset
+#define CFI_UNDEFINED	.cfi_undefined
+
+#else
+
+#define CFI_IGNORE	#
+
+#define CFI_STARTPROC	CFI_IGNORE
+#define CFI_ENDPROC	CFI_IGNORE
+#define CFI_DEF_CFA	CFI_IGNORE
+#define CFI_REGISTER	CFI_IGNORE
+#define CFI_REL_OFFSET	CFI_IGNORE
+#define CFI_UNDEFINED	CFI_IGNORE
+
+#endif	/* !ARC_DW2_UNWIND_AS_CFI */
+
+#endif	/* __ASSEMBLY__ */
+
+#endif	/* _ASM_ARC_DWARF_H */

arch/arc/include/asm/linkage.h

@@ -9,6 +9,8 @@
 #ifndef __ASM_LINKAGE_H
 #define __ASM_LINKAGE_H
 
+#include <asm/dwarf.h>
+
 #ifdef __ASSEMBLY__
 
 #define ASM_NL		 `	/* use '`' to mark new line in macro */
@@ -32,6 +34,16 @@
 #endif
 .endm
 
+#define ENTRY_CFI(name)		\
+	.globl name ASM_NL	\
+	ALIGN ASM_NL 		\
+	name: ASM_NL		\
+	CFI_STARTPROC ASM_NL
+
+#define END_CFI(name) 		\
+	CFI_ENDPROC ASM_NL	\
+	.size name, .-name
+
 #else	/* !__ASSEMBLY__ */
 
 #ifdef CONFIG_ARC_HAS_ICCM