startPerformIO: Only save C stack pointer when
compiling the conc RTS on x86s.
/* Save away C stack pointer so that we can restore it when we leave
the Haskell world.
*/
+#if defined(CONCURRENT) && defined(i386_TARGET_ARCH)
SpB -= BREL(1);
*SpB = (W_)SAVE_esp;
+#endif
/* Put a World State Token on the B stack */
/* This is necessary because we've not unboxed it (to reveal a void) yet */