powerpc: put the isync inside the TD_LOCK() checking loop

Fix a narrow window where the lock is unlocked but the checking CPU
hasn't flushed things appropriately.

Inside this window the CPU inside cpu_switch() will loop forever thinking
the destination thread is still blocked/locked even though it is not.

This manifests as the system hanging after starting all APs.

I've seen this reliably trigger in qemu-system-ppc64 running power9 pseries
guests; the more CPUs the more likely it triggers at boot.

PR: kern/292167
Differential Revision:	https://reviews.freebsd.org/D54478
Reviewed by:	jhibbits
MFC after:	1 week
Relnotes:	yes

sys/powerpc/powerpc/swtch32.S

@@ -141,8 +141,8 @@ cpu_switchin:
 blocked_loop:
 	lwz	%r7,TD_LOCK(%r2)
 	cmpw	%r6,%r7 
-	beq-	blocked_loop
 	isync
+	beq-	blocked_loop
 #endif
 
 	lwz	%r17,TD_PCB(%r2)	/* Get new current PCB */

sys/powerpc/powerpc/swtch64.S

@@ -194,8 +194,8 @@ cpu_switchin:
 blocked_loop:
 	ld	%r7,TD_LOCK(%r13)
 	cmpd	%r6,%r7 
-	beq-	blocked_loop
 	isync
+	beq-	blocked_loop
 #endif
 	
 	ld	%r17,TD_PCB(%r13)	/* Get new PCB */