Fixed #58 zdot SEGFAULT bug with GCC-4.6. Thank Mr. John for this patch.

In i386 calling convention, the caller put the address of return value of zdot into the first hidden parameter.
Thus, the callee should delete this address before return.
Actually, I have fixed the same bug on x86/zdot_sse2.S (issue #32). However, that is not a good implementation which uses 3 instructions. Mr. John told me used "ret $0x4" to skip the first hidden address (4 bytes).

kernel/x86/xdot.S

@@ -307,7 +307,11 @@
 	popl	%ebx
 	popl	%esi
 	popl	%edi
+#if defined(F_INTERFACE) && defined(RETURN_BY_STACK)
+	ret	$0x4
+#else
 	ret
+#endif
 	ALIGN_3
 
 .L88:
@@ -326,6 +330,10 @@
 	popl	%ebx
 	popl	%esi
 	popl	%edi
-	ret
 
+#if defined(F_INTERFACE) && defined(RETURN_BY_STACK)
+	ret	$0x4
+#else
+	ret
+#endif
 	EPILOGUE

kernel/x86/zdot.S

@@ -283,7 +283,11 @@
 	popl	%ebx
 	popl	%esi
 	popl	%edi
+#if defined(DOUBLE) || defined(XDOUBLE)
+	ret	$0x4
+#else
 	ret
+#endif
 	ALIGN_3
 
 .L88:
@@ -305,6 +309,10 @@
 	popl	%ebx
 	popl	%esi
 	popl	%edi
-	ret
 
+#if defined(DOUBLE) || defined(XDOUBLE)
+	ret	$0x4
+#else
+	ret
+#endif
 	EPILOGUE

kernel/x86/zdot_sse2.S

@@ -1542,7 +1542,5 @@
 	popl	%esi
 	popl	%edi
 /*remove the hidden return value address from the stack.*/
-	popl	%ecx
-	xchgl	%ecx, 0(%esp)
-	ret
+	ret	$0x4
 	EPILOGUE