[dev.power64] runtime: define page size on power64le systems to be 64k

Fixes #8495.

CL 128260043 updated the definition of syscall.GetPageSize to report 64k for power64 and power64le. This CL cleans up the last place where the page size was defined as 4k.

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://golang.org/cl/123520043

src/pkg/runtime/arch_power64le.h

@@ -7,7 +7,7 @@ enum {
 	BigEndian = 0,
 	CacheLineSize = 64,
 	RuntimeGogoBytes = 64,
-	PhysPageSize = 4096,
+	PhysPageSize = 65536,
 	PCQuantum = 4,
 	Int64Align = 8
 };