[ARM] Fix double compare with zero instructions.

Accessing register index -1 causes vfp_get_double() to enter an
infinite loop.  Use VFP_REG_ZERO (value 16) and provide code to
emulate a sixteenth register which always returns 0.0

arch/arm/vfp/vfp.h

@@ -249,6 +249,12 @@ struct vfp_double {
 	u64	significand;
 };
 
+/*
+ * VFP_REG_ZERO is a special register number for vfp_get_double
+ * which returns (double)0.0.  This is useful for the compare with
+ * zero instructions.
+ */
+#define VFP_REG_ZERO	16
 extern u64 vfp_get_double(unsigned int reg);
 extern void vfp_put_double(unsigned int reg, u64 val);
 

arch/arm/vfp/vfpdouble.c

@@ -427,12 +427,12 @@ static u32 vfp_double_fcmpe(int dd, int unused, int dm, u32 fpscr)
 
 static u32 vfp_double_fcmpz(int dd, int unused, int dm, u32 fpscr)
 {
-	return vfp_compare(dd, 0, -1, fpscr);
+	return vfp_compare(dd, 0, VFP_REG_ZERO, fpscr);
 }
 
 static u32 vfp_double_fcmpez(int dd, int unused, int dm, u32 fpscr)
 {
-	return vfp_compare(dd, 1, -1, fpscr);
+	return vfp_compare(dd, 1, VFP_REG_ZERO, fpscr);
 }
 
 static u32 vfp_double_fcvts(int sd, int unused, int dm, u32 fpscr)

arch/arm/vfp/vfphw.S

@@ -199,6 +199,11 @@ vfp_get_double:
 	mov	pc, lr
 	.endr
 
+	@ virtual register 16 for compare with zero
+	mov	r0, #0
+	mov	r1, #0
+	mov	pc, lr
+
 	.globl	vfp_put_double
 vfp_put_double:
 	mov	r0, r0, lsr #1