selftests/powerpc: Ensure 16-byte stack pointer alignment

The PUSH/POP_BASIC_STACK helpers in basic_asm.h do not ensure that the
stack pointer is always 16-byte aligned, which is required per the ABI.

Fix the macros to do the alignment if the caller fails to.

Currently only one caller passes a non-aligned size, tm_signal_self(),
which hasn't been caught in testing, presumably because it's a leaf
function.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220627140239.2464900-1-mpe@ellerman.id.au

tools/testing/selftests/powerpc/include/basic_asm.h

@@ -58,7 +58,7 @@
 #define PUSH_BASIC_STACK(_extra) \
 	mflr	r0; \
 	std	r0, STACK_FRAME_LR_POS(%r1); \
-	stdu	%r1, -(_extra + STACK_FRAME_MIN_SIZE)(%r1); \
+	stdu	%r1, -(((_extra + 15) & ~15) + STACK_FRAME_MIN_SIZE)(%r1); \
 	mfcr	r0; \
 	stw	r0, STACK_FRAME_CR_POS(%r1); \
 	std	%r2, STACK_FRAME_TOC_POS(%r1);
@@ -67,7 +67,7 @@
 	ld	%r2, STACK_FRAME_TOC_POS(%r1); \
 	lwz	r0, STACK_FRAME_CR_POS(%r1); \
 	mtcr	r0; \
-	addi	%r1, %r1, (_extra + STACK_FRAME_MIN_SIZE); \
+	addi	%r1, %r1, (((_extra + 15) & ~15) + STACK_FRAME_MIN_SIZE); \
 	ld	r0, STACK_FRAME_LR_POS(%r1); \
 	mtlr	r0;