Cygwin: gendef: fix AArch64 stabilize_sig_stack lock retry

The retry label sat below the code that computes the stacklock address
and the "locked" constant, so those were set up once on entry and not
recomputed.  The signal-handler path below branches back to the label
after using x9 and x12 for the incyg counter, so a retry could operate
on incyg rather than stacklock, or spin.

Move the label above the setup so both are recomputed on every
iteration, and clear the exclusive monitor before yielding on
contention.

Fixes: 5effc7a9bf ("Cygwin: gendef: Implement stabilize_sig_stack for aarch64")
Signed-off-by: Aswin Kalies <aswin.kalies@multicorewareinc.com>

winsup/cygwin/scripts/gendef

@@ -502,11 +502,13 @@ stabilize_sig_stack:
 
 	ldr	x10, [x18, #0x8]		// load TLS block base pointer into x10
 
-	// try to acquire the lock
+	// try to acquire the lock.  The address and the "locked" constant are
+	// recomputed on every iteration: the signal-handler path below branches
+	// back here after using x9/x12 for the incyg counter.
+1:
 	mov	w9, #1				// value to store (1 == locked)
 	ldr	x11, =_cygtls.stacklock		// load the symbol offset
 	add	x12, x10, x11			// x12 = tls_base + &stacklock
-1:
 	ldaxr	w13, [x12]			// load old lock value
 	cbnz	w13, 2f				// if already locked, wait
 	stxr	w14, w9, [x12]			// attempt to acquire the lock
@@ -514,6 +516,7 @@ stabilize_sig_stack:
 	b	3f				// lock acquired, continue
 
 2:
+	clrex					// drop the exclusive monitor
 	yield					// yield to allow other threads to run
 	b	1b				// retry acquiring the lock