Sparc: use the FLUSHW instruction when available

This instruction tends to have better performance than

    ta ST_FLUSHWINDOWS

It isn't available on all systems though, hence the conditional.

Signed-off-by: Giel van Schijndel <me@mortis.eu>

platform/switch_sparc_sun_gcc.h

@@ -55,7 +55,11 @@ slp_switch(void)
      *
      * Then put the stack pointer into stackref. */
     __asm__ volatile (
+#ifdef __sparcv9
+        "flushw\n\t"
+#else
         "ta %1\n\t"
+#endif
         "mov %%sp, %0"
         : "=r" (stackref) :  "i" (ST_FLUSH_WINDOWS));