sh: smp: shove a cpu_relax() in the plat_start_cpu() busy loop.

Without this, certain versions of GCC will happily optimize the entire
loop out.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/kernel/cpu/sh4a/smp-shx3.c

@@ -82,7 +82,7 @@ void plat_start_cpu(unsigned int cpu, unsigned long entry_point)
 		ctrl_outl(STBCR_MSTP, STBCR_REG(cpu));
 
 	while (!(ctrl_inl(STBCR_REG(cpu)) & STBCR_MSTP))
-		;
+		cpu_relax();
 
 	/* Start up secondary processor by sending a reset */
 	ctrl_outl(STBCR_AP_VAL, STBCR_REG(cpu));