aarch64: Fix BTI support in libitm

sjlj.S did not have the GNU property note markup and the BTI c
instructions that are necessary when it is built with branch
protection.

The notes are only added when libitm is built with branch
protection, because old linkers mishandle the note (merge
them incorrectly or emit warnings), the BTI instructions
are added unconditionally.

2020-07-09  Szabolcs Nagy  <szabolcs.nagy@arm.com>

libitm/ChangeLog:

	* config/aarch64/sjlj.S: Add BTI marking and related definitions,
	and add BTI c to function entries.

(cherry picked from commit 319078dad62eba942d33c8975bdcbb09d1c68ba6)

libitm/config/aarch64/sjlj.S

@@ -24,6 +24,8 @@
 
 #include "asmcfi.h"
 
+#define BTI_C	hint	34
+
 	.text
 	.align	2
 	.global	_ITM_beginTransaction
@@ -31,6 +33,7 @@
 
 _ITM_beginTransaction:
 	cfi_startproc
+	BTI_C
 	mov	x1, sp
 	stp	x29, x30, [sp, -11*16]!
 	cfi_adjust_cfa_offset(11*16)
@@ -70,6 +73,7 @@ GTM_longjmp:
 	/* The first parameter becomes the return value (x0).
 	   The third parameter is ignored for now.  */
 	cfi_startproc
+	BTI_C
 	ldp	x19, x20, [x1, 1*16]
 	ldp	x21, x22, [x1, 2*16]
 	ldp	x23, x24, [x1, 3*16]
@@ -87,6 +91,29 @@ GTM_longjmp:
 	cfi_endproc
 	.size	GTM_longjmp, . - GTM_longjmp
 
+/* GNU_PROPERTY_AARCH64_* macros from elf.h for use in asm code.  */
+#define FEATURE_1_AND 0xc0000000
+#define FEATURE_1_BTI 1
+#define FEATURE_1_PAC 2
+
+/* Add a NT_GNU_PROPERTY_TYPE_0 note.  */
+#define GNU_PROPERTY(type, value)	\
+  .section .note.gnu.property, "a";	\
+  .p2align 3;				\
+  .word 4;				\
+  .word 16;				\
+  .word 5;				\
+  .asciz "GNU";				\
+  .word type;				\
+  .word 4;				\
+  .word value;				\
+  .word 0;
+
 #if defined(__linux__) || defined(__FreeBSD__)
 .section .note.GNU-stack, "", %progbits
+
+/* Add GNU property note if built with branch protection.  */
+# ifdef __ARM_FEATURE_BTI_DEFAULT
+GNU_PROPERTY (FEATURE_1_AND, FEATURE_1_BTI)
+# endif
 #endif