runtime: ensure that asmsyscall6 follow AIX stack convention

The function asmsyscall6 must follow AIX stack layout. It means
that its first local variable must be stored after its arguments
area, ie after offset 112.

Fixes #50185

Change-Id: I897731ddd2a9faad8218443a4c2f4b204ad7e173
Reviewed-on: https://go-review.googlesource.com/c/go/+/373074
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/sys_aix_ppc64.s

@@ -25,7 +25,12 @@ TEXT callCfunction<>(SB),	NOSPLIT|NOFRAME,$0
 // stored in libcall_fn and store the results in libcall struture
 // Up to 6 arguments can be passed to this C function
 // Called by runtime.asmcgocall
-// It reserves a stack of 288 bytes for the C function.
+// It reserves a stack of 288 bytes for the C function. It must
+// follow AIX convention, thus the first local variable must
+// be stored at the offset 112, after the linker area (48 bytes)
+// and the argument area (64).
+// The AIX convention is described here:
+// https://www.ibm.com/docs/en/aix/7.2?topic=overview-runtime-process-stack
 // NOT USING GO CALLING CONVENTION
 // runtime.asmsyscall6 is a function descriptor to the real asmsyscall6.
 DATA	runtime·asmsyscall6+0(SB)/8, $asmsyscall6<>(SB)
@@ -34,7 +39,8 @@ DATA	runtime·asmsyscall6+16(SB)/8, $0
 GLOBL	runtime·asmsyscall6(SB), NOPTR, $24
 
 TEXT asmsyscall6<>(SB),NOSPLIT,$256
-	MOVD	R3, 48(R1) // Save libcall for later
+	// Save libcall for later
+	MOVD	R3, 112(R1)
 	MOVD	libcall_fn(R3), R12
 	MOVD	libcall_args(R3), R9
 	MOVD	0(R9), R3
@@ -50,7 +56,7 @@ TEXT asmsyscall6<>(SB),NOSPLIT,$256
 	MOVD	40(R1), R2
 
 	// Store result in libcall
-	MOVD	48(R1), R5
+	MOVD	112(R1), R5
 	MOVD	R3, (libcall_r1)(R5)
 	MOVD	$-1, R6
 	CMP	R6, R3