runtime: save and restore CR for ppc64le

C code expects CR2, CR3, and CR4 to be preserved across function calls.
Preserve the entire CR register across function calls in
_rt0_ppc64le_linux_lib and crosscall2. The standard ppc64le call frame
uses 8(R1) as the place to save CR; emulate that.

It's hard to write a reliable test for this as it requires writing C
code that sets CR2, CR3, or CR4 across a call to a Go function.

Change-Id: If39e771a5b574602b848227312e83598fe74eab7
Reviewed-on: https://go-review.googlesource.com/44733
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>

src/runtime/cgo/asm_ppc64x.s

@@ -16,6 +16,8 @@ TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 	// Start with standard C stack frame layout and linkage
 	MOVD	LR, R0
 	MOVD	R0, 16(R1)	// Save LR in caller's frame
+	MOVW	CR, R0		// Save CR in caller's frame
+	MOVD	R0, 8(R1)
 	MOVD	R2, 24(R1)	// Save TOC in caller's frame
 
 	BL	saveregs2<>(SB)
@@ -38,6 +40,8 @@ TEXT crosscall2(SB),NOSPLIT|NOFRAME,$0
 	BL	restoreregs2<>(SB)
 
 	MOVD	24(R1), R2
+	MOVD	8(R1), R0
+	MOVFL	R0, $0xff
 	MOVD	16(R1), R0
 	MOVD	R0, LR
 	RET

src/runtime/rt0_linux_ppc64le.s

@@ -8,6 +8,8 @@ TEXT _rt0_ppc64le_linux_lib(SB),NOSPLIT,$-8
 	// Start with standard C stack frame layout and linkage.
 	MOVD	LR, R0
 	MOVD	R0, 16(R1) // Save LR in caller's frame.
+	MOVW	CR, R0     // Save CR in caller's frame
+	MOVD	R0, 8(R1)
 	MOVD	R2, 24(R1) // Save TOC in caller's frame.
 	MOVDU	R1, -320(R1) // Allocate frame.
 	
@@ -120,6 +122,8 @@ done:
 
 	ADD	$320, R1
 	MOVD	24(R1), R2
+	MOVD	8(R1), R0
+	MOVFL	R0, $0xff
 	MOVD	16(R1), R0
 	MOVD	R0, LR
 	RET