runtime: fix wrong offset when calling ppc64x nanotime syscall

There is a wrong offset when getting the results of a clock_gettime
syscall. Although the syscall will never be called in native ppc64x,
QEMU doesn't implement VDSO, so it will return wrong values.

Fixes #36592

Change-Id: Icf838075228dcdd62cf2c1279aa983e5993d66ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/215397
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

Author: ceseo

src/runtime/sys_linux_ppc64x.s

@@ -280,7 +280,7 @@ fallback:
 	ADD	$32, R1, R4
 	SYSCALL $SYS_clock_gettime
 	MOVD	32(R1), R3
-	MOVD	48(R1), R5
+	MOVD	40(R1), R5
 	JMP	finish
 
 TEXT runtime·rtsigprocmask(SB),NOSPLIT|NOFRAME,$0-28