Fix booting with >4GB RAM on PowerMac G5 hardware

===
From Nathan Whitehorn:

Open Firmware runs in virtual mode on the Powermac G5. This runs inside the
kernel page table, which preserves all address translations made by OF before
the kernel starts; as a result, the kernel address space is a strict superset of
OF's.

Where this explodes is if OF uses an unmapped SLB entry. The SLB fault handler
runs in real mode and refers to the PCPU pointer in SPRG0, which blows up the
kernel. Having a value of SPRG0 that works for the kernel is less fatal than
preserving OF's value in this case.

===

The result of this is seemingly random panics from NULL dereferences, or hangs
immediately upon boot.  By not restoring SPRG0 for Open Firmware entry the
kernel PCPU pointer is preserved and SLB faults are successful, resulting in a
stable kernel.

PR:		205458
Reported by:	several (over bugzilla, lists, IRC)
Reviewed by:	andreast
Tested by:	many (various forms)
MFC after:	2 weeks

sys/powerpc/ofw/ofw_machdep.c

@@ -111,6 +111,15 @@ ofw_sprg_prepare(void)
 	 * Assume that interrupt are disabled at this point, or
 	 * SPRG1-3 could be trashed
 	 */
+#ifdef __powerpc64__
+	__asm __volatile("mtsprg1 %0\n\t"
+	    		 "mtsprg2 %1\n\t"
+			 "mtsprg3 %2\n\t"
+			 :
+			 : "r"(ofmsr[2]),
+			 "r"(ofmsr[3]),
+			 "r"(ofmsr[4]));
+#else
 	__asm __volatile("mfsprg0 %0\n\t"
 			 "mtsprg0 %1\n\t"
 	    		 "mtsprg1 %2\n\t"
@@ -121,6 +130,7 @@ ofw_sprg_prepare(void)
 			 "r"(ofmsr[2]),
 			 "r"(ofmsr[3]),
 			 "r"(ofmsr[4]));
+#endif
 }
 
 static __inline void
@@ -136,7 +146,9 @@ ofw_sprg_restore(void)
 	 *
 	 * PCPU data cannot be used until this routine is called !
 	 */
+#ifndef __powerpc64__
 	__asm __volatile("mtsprg0 %0" :: "r"(ofw_sprg0_save));
+#endif
 }
 #endif
 
@@ -344,8 +356,9 @@ OF_initial_setup(void *fdt_ptr, void *junk, int (*openfirm)(void *))
 	ofmsr[0] = mfmsr();
 	#ifdef __powerpc64__
 	ofmsr[0] &= ~PSL_SF;
-	#endif
+	#else
 	__asm __volatile("mfsprg0 %0" : "=&r"(ofmsr[1]));
+	#endif
 	__asm __volatile("mfsprg1 %0" : "=&r"(ofmsr[2]));
 	__asm __volatile("mfsprg2 %0" : "=&r"(ofmsr[3]));
 	__asm __volatile("mfsprg3 %0" : "=&r"(ofmsr[4]));