Inline 'if' in GC instrumentation

This is the same version as proposed by Evgeny, only changing the actual
suspension function name back to where it was before to be able to perform less
changes in libgc.

Instead of
__nacl_suspend_thread_if_needed ();

instrumentation now looks like
if (__nacl_thread_suspension_needed)
  __nacl_suspend_thread_if_needed ();

GC runtime should define
volatile int __nacl_thread_suspension_needed;
void __nacl_suspend_thread_if_needed (void);



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