Add a warning when compiling on a platform where imp_implementationWithBlock() won't work.

block_trampolines.S

@@ -46,12 +46,13 @@ __objc_block_trampoline_sret:
 next_line2:
 	pop   %eax                 # Load the old instruction pointer
 	mov   8(%esp), %ebx        # Load the self parameter
-	mov   %ebx, 12(%esp)        # Store self as the second argument
+	mov   %ebx, 12(%esp)       # Store self as the second argument
 	mov   -9(%eax), %ebx       # Load the block pointer to %ebx
 	mov   %ebx, 8(%esp)        # Store the block pointer in the first argument
 	jmp   *-13(%eax)           # Call the block function
 __objc_block_trampoline_end_sret:
 #else
+#warning imp_implementationWithBlock() not implemented for your architecture
 __objc_block_trampoline:
 __objc_block_trampoline_end:
 __objc_block_trampoline_sret: