Make fiber_pool more conservative on platforms with limited address space.

We use COROUTINE_LIMITED_ADDRESS_SPACE to select platforms where address
space is 32-bits or less. Fiber pool implementation enables more book
keeping, and reduces upper limits, in order to minimise address space
utilisation.

coroutine/x86/Context.h

@@ -11,6 +11,7 @@
 #include <string.h>
 
 #define COROUTINE __attribute__((noreturn, fastcall)) void
+#define COROUTINE_LIMITED_ADDRESS_SPACE
 
 enum {COROUTINE_REGISTERS = 4};