Cygwin: gendef: Implement sigsetjmp for AArch64

Co-authored-by: Radek Bartoň <radek.barton@microsoft.com>

Signed-off-by: Thirumalai Nagalingam <thirumalai.nagalingam@multicorewareinc.com>

Author: thiru-mcw

winsup/cygwin/scripts/gendef

@@ -607,7 +607,28 @@ EOF
 	# TODO: These are only stubs, they need to be implemented properly for AArch64.
 	return <<EOF;
 	.globl	sigsetjmp
+	.seh_proc sigsetjmp
 sigsetjmp:
+	// prologue
+	stp		fp, lr, [sp, #-0x10]!	// save FP and LR registers
+	mov		fp, sp			// set FP to current SP
+	.seh_endprologue
+	str	w1, [x0, #0x100]		// buf->savemask = savemask
+	cbz     w1, 1f				// If savemask == 0, skip fetching sigmask
+	mov     x3, x0                        	// save buf in x3
+	sub     sp, sp, #32			// Allocate 32 bytes on stack call
+	mov     x0, #0                         	// SIG_SETMASK
+	mov     x1, xzr                        	// newmask = NULL
+	add     x2, x3, #0x108                 	// &buf->sigmask
+	bl      pthread_sigmask
+	add     sp, sp, #32
+1:
+	bl	setjmp
+	// epilogue
+	ldp	fp, lr, [sp], #0x10		// restore saved FP and LR registers
+	ret
+	.seh_endproc
+
 	.globl  setjmp
 setjmp:
 	.globl	siglongjmp